18
N. U. STEPHENSON
: OBSERVATIONS ON THE
OBSERVATIONS ON THE DEVELOPMENT OF THE AMPHICOELOUS
FROGS, LEIOPELMA AND ASCAPHUS. BY N. G. STEPHENSON.
(Communicated by Prof. G . R. DE BEER,F.R.S., P.L.S.)
(PLATES
1-3.)
[Head 20 January 1949.1
The two genera Leiopelma (New Zealand) and Asmphus (Northwestern United
States of America), the most primitive of living frogs, were placed by Noble (193 1) in
the sub-order Amphicoela. Previously (1922) he had referred both genera to the
Discoglossidae, but in 1924 he proposed a new family, the Liopelmidae* for their
reception, and in his later classification of the Amphibia (1931) he placed the Liopelmidae in a separate sub-order a t the base of his evolutionary tree for the Anura. He
justified this classification on the grounds that these primitive genera are distinguished
from all other frogs by their amphicoelous vertebrae, the interdorsals and interventrals
remaining cartilaginous as in the majority of urodeles. He also noted that both
genera are primitive in possessing two tail-wagging muscles, the pyriformis and the
caudali-pubo-ischio-tibialis,even though neither possesses a tail in the adult. Noble’s
classification has been upheld as far as the primitiveness of the two genera is
concerned by subsequent investigations of the adult morphology. But although the
importance of studies on the development of these frogs was stressed by de Beer (1937)
and Pusey (1938), little information is as yet available apart from some general
observations on their life histories.
The first general account of the life history of Leiopelma was givenby Archey (1922),
whose description was based on observations of developing eggs, some of which were
hatched out in the laboratory. Some of his material was re-examined by Noble (1927)
who noted a close resemblance between the early embryos of Leiopelma and Aecaphus.
Observations on the life history of Aecaphus were published by Gaige (1920) and Noble
and Putnam (1931). The first detailed account of structures in any developmental
stage of either genus was given by Pusey (1943) in his account of the chondrocranium,
jaws, arches and muscles of a partly-grown larva of Asmphus.
Detailed investigations of development in these primitive frogs has been delayed
through difficulties of obtaining material for research. This is particularly true of
Leiopelma, which is only found in rather inaccessible localities and is restricted in its
distribution ; it is protected in New Zealand under the Animals Proteotion and Game
Act, 1921-22. The following description and discussion are based on specimens of
Leiopelma in various stages of development, which were colleoted from the Coromandel
Peninsula, by permission of the New Zealand Government. A valuable series of
metamorphosing tadpoles of Asmplaus true& obtained from Mt. Rainier, Washington,
waa provided by the Auokland War Memorial Museum.
The life histories of these two frogs maybesummarizedasfollows:whereasAecuphus
lays its eggs in water and has an aquatic tadpole stage, Leiopelm has been observed
to lay eggs on land only and these eggs undergo intracapsular development ; there is n o
free-living aquatic tadpole stage, and a tailed froglet hatches from the egg. It might
be thought that development on land would involve striking specializations. This is
not the oase and such adaptations as ocour are related to intracapsular rather than
terrestrial conditions of life. It is evident from a number of examples among
urodeles and frogs that amphibian eggs which undergo intracapsular development in
water do not require fundamental structural adaptations t o enable them to undergo
* As Turbott (1942)pointed out, Fitzinger’s (1881)original spelling, viz. LEIOPELMA, should
be retained. The spelling Liopelma first appeared in a deeoription by Gkther (1888) of tailless
batrmhiana added to the collection of the BritishMuseum. Furthermore, Noble’sfamilirtlname WBB
inoorrectly formed from the root and should be amended to LEIOPELMATIDAE. Noble (1942)
stated in a footnote that he followed present day custom in using the oldest generic name in forming
the familial name.
19
DEVELOPMENT OF THE AMPHICOELOUS PROGS
such development in damp situations on land, and vice versa. With Amphibia, the
descriptive term ‘ development on land ’ is perhaps misleading, for it does not mean
complete independence of surface water.
The eggs of Leiopelma have been noted in large numbers in areas of sphagnum bog
at Mt. Moehau, Coromandel Peninsula, N.Z., in damp situations which, though essentially terrestrial, could be subject a t least to temporary inundation through heavy rains.
A few of these eggs* were collected for embryological study and all proved to be those
of the recently described terrestrial type, L. archeyi (Turbott, 1942). Although gravid
females of the more aquatic L.hochetetteri were found a t Coromandel with L.archeyi in
terrestrial areas during the breeding season, no evidence of egg-laying habits was
obtained from the Huia and Warkworth areas, both near Auckland, where L. hochstetteyi alone occurs. Thus one cannot dismiss the possibility that the eggs of L .
hochsteftt-vi might be laid for preference in water and there undergo intracapsular
development.
Both Leiopelma and Ascuphus lay large, yolky, unpigmented eggs. The freshly
laid egg of Ascuphue with its capsules measures 4.5 mm., the egg alone 4 mm.. according
to Noble and Putnam (1931). Gaige’s (1920) measurements are capsule, d mm.,
yolk, 5 mm. diameter. The eggs of Leiopelma are of similar size ; a t the time of laying
they are approximately 5 mm. in capsular diameter. As development proceeds in
h i o p e h a , there is a considerable accumulation of fluid around the embryo, and the
enveloping capsule may be distended to a diameter of about 1 cm. before the time of
hatching. The embryo within reaches a total length of 17 mm. to 19 mm. and its tail
is curved from the base around the side of the body, so that the tip is close to the head,
or even projects in front of it. (Pl. 1,fig. 4.)
Aocording to Noble and Putnam (1931), each egg of Ascuphus is surrounded by
two capsules and a very thin vitelline membrane. The outer capsule is very adhesive
80 that the eggs often stick together in rods or clumps, but there is no enveloping
membrane for the cluster, as described by Archey (1922) in Leiopelmu. Archey
states that the eggs of Leiopelma are laid in clusters and that each cluster is surrounded
by a soft transparent gelatinous envelope, the outer portion of which forms a thin
membrane. I could not confirm the occurrence of a n enveloping membrane, and found
that the capsular arrangements are essentially similar to those described in A ecaphue.
Immediately within the gelatinous capsule, but closely applied to it, is a thin but
distinct vitelline membrane which is sharply differentiated from the capsular jelly by
its staining properties. Some adult specimens of Leiopelm, brought from New
Zealand in August 1947,have since been kept alive and in healthy condition by my wife
in the Department of Zoology, University College, London. Two of these animals
were induced to lay eggs, which proved to be infertile. In one instance, the eggs were
observed to be laid in a single rosary-like string, but, when the frog was disturbed,
the eggs rolled together into a cluster and adhered firmly to one another, because the
outer layer of the gelatinous capsule is very stioky at first.
I n field observations at Mt. Moehau, Coromandel, N.Z., it was noted that the
number of eggs laid by Leiopelma may range from two t o eight in a cluster. It is
clear from dissections that the eggs from each ovary are laid separately, and that the
eggs of a cluster represent the number of mature eggs in one ovary at the time of
laying. The ovaries contain large and small eggs a t all times of the year, 80 that
presumably the maturation period is a lengthy one. Ascaphus has been induced to
* In the laboratory, the eggs of Leiopelma were hatched out from clusters which were placed on
d m p mose in dishes with about a third of an inch of water in the bottom of the dish. Eggs am
often attacked by moulds and in order to prevent such growths from becoming established on the
outside of the capsules, the clusters were picked up and washed daily under a stream from the ta
After they hatched, the tailed froglets, about 20 mm. in total length, were also examined da&
and were seen to be rather quiescent,being found either on the damp moss or on the bottom ofthe
dish completely submerged in water. Occasionally one would craw1 up the vertical sides of the
glass dish. The froglets swam, when disturbed, by alternate movements of the limbs as well as
by movementsof the tail. A t first, recently hatched frogs merely crawled about on t.hemoss, bu3
after two weeke they were capable of jumping in leaps of two or three inches.
.
2*
20
N. 0 . STEPHENSON
:
OBSERVATIONS ON THE
lay eggs in the laboratory (Noble and Putnam, 1931) by the implantation of fresh
anterior pituitary substance of various Amphibia. The number of eggs laid under
these conditions has varied from 28 t o 47 and most of them developed normally.
I n Ascaphus, fertilization is internal. The genus is unique among Anura in that
the male frog possesees a cloaca1appendage-a ' phallic organ ' or tail '-which serves
as a copulatory organ. DeVilliers (1934)compared this intromittent organ of Ascaphw,
with the copulatory structures described in urodeles (Triton) and in the Gymnophiona
,(iSZiphmmpa). The possession of a copulatory organ by Ascaphus constitutes a striking
difference between Aecaphw and Leiopelma, in which genus no such structure occurs.
No direct evidence regarding fertilization in Leiopelma is available a t present, but
amplexus, which is of the lumbar type in Aecuphus, had never been seen in Leiopelmu,
although this frog has bean observed in the field for several years, and although
specimens have been kept in captivity for over a year. Brooding habits, however, do
occur in Leiopelma and it is usual to find an adult male sitting over the cluster of eggs.
Since no one has observed Leiopelma laying eggs under natural conditions, and
attempts t o obtain fertile eggs under laboratory conditions have not been successful,
one can only make an approximate estimate of developmental times. I n other
Amphibia which lay eggs in clusters, the individual eggs vary in their developmental
rate. Those on the outside of the cluster develop more rapidly, and are the f i s t to hatch.
I n Leiopelmu, the number of eggs in each cluster is very small compared with the
number in other Amphibia and a considerable surface of each egg is exposed. It is
known that whereas the eggs of some clusters may hatch within a few hours of each
other, the hatching of eggs of large clusters may extend over a few days. It is notable,
however, that commonly one or more eggs in a cluster may fail to show any sign of
development. Eggs of Leiopelmu have been observed in the field during the months of
November and December. Egg clusters were noted on 23 November 1946, a t Mt.
Moehau. The f i s t eggs of these clusters hatched on 25 December 1946, and the
last on 3 January 1947. Thus, the youngest intracapsular embryos hatched 41 days
after the first being observed and there was a difference of 9 days between the times of
hatching of the first and last eggs of these clusters. Without considering variation in
the developmental rate, which could only make a difference of a few days, one may
estimate that the frog developsas an intracapsular embryo for at least six weeks, possibly
longer, since even the youngest eggs may not have been laid immediately prior t o
discovery. After hatching, the tail is absorbed and has practically disappeared a t the
,end of the fourth week.
Archey (1922) found that an intracapsular embryo of Leiopelmu, prematurely
extracted from its capsule and placed in water in a watch glass, straightened itself out
and continued to develop. Although its rate of development was much slower than
that of individuals inside their capsules, it continued to develop for 11 days and then
was accidentally destroyed.
Noble and Putnam (1931) noted that the larva of Ascaphus does not hatch until
a month after the egg is laid in water. At this time the larva measures approximately
13-5 mm. total length, 5 mm. in head and body length. The tadpole shows considerable adaptation to life in mountain torrents. Despite these mountain brook adaptations, which have no counterpart in Leiopelma, Noble (1927) noted that there was
a close resemblance between the early intracapsular embryos of both genera.
It seams likely from the field notes of Putnam, quoted by Noble (1937), that the
eggs of Aecaphus hatch during August and tk,at the tadpoles metamorphose in August
or September of the following year. I n the series of photographs of tadpoles which
illustrate stages in this metamorphosis (Pl.3, figs. 2, 3 & 4), it may be seen that the
tadpole is provided with a complete branchial chamber, closed except for a single
median spiracle which is very inconspiouous and is placed immediately behind the
lower lip that often conceals it. The developing fore limbs are thus housed in the
branchial chamber. The emergence of the fore limbs a t metamorphosis, unlike that
of R a m , is symmetrical in that they both project through the single median spiracle
(Pl. 3, fig. 3). The hind limbs of Aecaphus are visible externally before the fore
DEVELOPMENTOFTIIEAMPHICOELOUS FROGS
21
limbs emerge and their tips are first seen projecting from beneath a large and prominent
flap, which also covers the anal papilla (Pl. 3, fig. 2). This flap is attached to the
body by its base, and to the tail by a thin membrane. As the hind limbs develop, i t
is pushed forwards and disappears.
Archey (1922)noted that in the intracapsular embryo of Leiopelma a transverse fold
or collar extended across the throat. He considered that its function might be
respiratory, but observed that it was not vascular a t any stage of development. This
gular fold is continuous from side to side and laterally, where it covers the bases of the
fore limbs, it becomes forwardly directed (Pl. 1). I find that this is essentially an
opercular fold which does not close to form a branchial chamber but remains widely
open from side to side. Thus there is, between the gular fold and the ventral body
wall, which is distended by the yolk endoderm, a deep branchial groove into which
visceral clefts open rather laterally. Internal gills are not developed in association
with these clefts, of which four are open on each side in the intracapsular embryo.
These clefts are gradually reduced in number, three remaining open on each side a t
the time of hatching, and one week after hatching a t least one cleft on each side can be
followed out to open to the exterior. At this stage also, the branchial groove has been
much reduced in the middle line, but it remains more distinct laterally where the gular
fold still covers the base of each fore limb. A spiracular pouch or cleft, which, as in all
Amphibia except the Gymnophiona, does not perforate the pharyngeal wall, is also
developed in the intracapsular embryo. This spiracular cleft is well developed up to
the t,ime of hatching, when it disappears. Thus, Eustachian tubes are not developed
and this is also the case in all urodeles, and in Ascaphus, Bombinu and certain other
anuran genera.
In its four open clefts devoid of internal gills, its opercular fold, and its widely open
branchial groove, Leiopelma has no parallel among the Anura. Closure of the visceral
clefts and reduction in their number, or closure of the spiracle and reduotion of the
closed branchial chamber are characteristic of other Anura with direct development
or non-aquatic tadpoles. Thus, although four pharyngeal pouches have been observed
in the direct development of Eleutheroductylus (Sampson, 1904 ; Giltin, 1944 ; three
pouches in E . nubicola, Lynn, 1942), these, as also in Anhydrophryne (Warren, 1922)
never perforate the pharyngeal wall. There is no branchial chamber in Eleutheroductylue, but, as first observed by Sampson (1904),separate dermal folds cover the base
of the fore limb on each side. Sampson noted that the dermal fold resembled an early
stage of an operculum, but was uncertain regarding its homology. Similar separate
dermal folds have been described in the development of Anhydrophryne rattrayi
(Warren, 1922). I n this latter species,the folds grow obliquely forwards and downwards
over the fore limb buds, and fuse on to the epidermis in front of and below the buds.
Thus the fore limbs continue to develop each in a separate enclosed cavity and do not
perforate the covering fold until late in metamorphosis. Warren states : ' It is not
possible to regard these fold8 as non-homologous with a true opercular fold, but the
line of origin of the fold has been shifted away from its typical position.' Lynn (1942),
however, was unable to support this homology from a study of the dermal folds in
Eleutherodactylus nubicola. I n both Eleutherodactylus and Anhydrophryne, the absence
of open clefts and the fact that the dermal folds are separate on each side have led to
difficulties in establishing such a homology.
In the development of the ranid Arthroleptella (de Villiers, 1929), which has a
non-aquatic tadpole, the pharynx is perforated only by a pair of openings, one on
each side, which are devoid of gills. These two openings place the pharynx in
communication with a chamber which is continuous from side to side but which has
no outlet to the exterior, for a spiracle is not developed. Similar conditions are
described for Breviceps parvus (de Villiers, 1929) and Sooglossua (Arthroleptis)
seychellensis (Brauer, 1898).
I n general, the only condition comparable to that of Leiopelma is to be found among
the urodeles, which may possess a similar fold and groove and similar, though fewer,
open clefts. Internal gills are never developed in association.with these clefts and the
22
N. 0. STEPHENSON
:
OBSERVATIONS ON THE
gular or opercular fold never grows back to form a branchial chamber. I n the tadpole
of Ascaphus, however, internal gills do occur, but no external gills are formed (Noble
and Putnam, 1931). The branchial arches are covered over by the opercular fold of
each side while they are still devoid of gill tufts.
The tail of the embryo of Leiopelma is neither flattened nor expanded and is strongly
muscular. Because of this, it is able to play an important role in the hatching process.
Archey (1922) has reported that hatching is a slow process correlated with a disintegration of the capsule, and Noble (1 926) remarks that the whole process seems more
urodele than frog-like. On several occasions my wife and I have observed this
hatching process, and we have noted that it is effected by vigorous movements of the
muscular tail, which breaks through the capsule. After the capsule is broken, the
embryo often remains with the tail, now straightened out, projecting from the
gelatinous mass. Occasionally the head also projects but the embryo may remain
for some time partially embedded in gelatinous material before wriggling clear.
When sections of the head of an intracapsular embryo are examined, it is found
that large numbers of gland cells occur dorsally in the epidermis. These cells, like
those described in Alytes by Noble (1926) and referred to below, are not restricted to a
single glandular area, but are widely scattered. In Heidenhain’s azan such cells are
strongly stained by the aniline blue. They are very numerous and cover a continuous
and considerable area over the head, dorsally between the eyes, and a narrow band
along the full length of the back in the mid-dorsal line. Anteriorly, they extend
dorsally over the snout and laterally around to the level of the external nostrils. At
this level they suddenly end and are neither found ventro-medianly below the nostrils,
nor on the ventral surface of the head generally. I n a transverse section of the head,
under low power magnifications, one may count over 100 secretory cells, superficially
placed in the epidermis, in the dorsal band. Along the back, as far as the beginning of
the tail, about 20 cells may be seen in any one transverse section. These secretory
cells, though characteristic of late intracapsular embryos, are not evident in the posthatching stages which have been examined. It is thus very probable that these
secretory cells have a function complementary to that of the muscular tail in the
hatching process of Leiopelma.
Noble ( 1926),in discussing the hatching processin Alytes, Eleutheroductylusandother
amphibians, considered the mechanisms whereby the amphibian embryo enclosed
within its egg capsule is able to free itself, and pointed out that the process is adequately
described in only a few forms (cf. Bles, 1905). He showed that in Alytes eosinophil
secretory cells, remarkably similar t o the cells of the frontal organ of R a m and Bufo,
initiate the hatching process. These cells, however, are scattered over the head and
snout and do not form a distinct frontal organ. Hatching ferments are not known to
occur in urodeles, but their possible occurrence has not been adequately investigated.
Noble examined sections of recently hatched Dearnognuthus fuscus and could not find
secretory cells comparable t o those in Alytea, but it must be noted that such secretory
cells have disappeared from the epidermis in recently hatched Leiopelma. The intracapsular embryo of urodeles should be examined for the presence or absence of these
cells.
The tail of Leiopelma is not only muscular, but also vascular. This condition in
.the living embryo is readily seen through the capsule, since pigment is not developed
in the tail. Archey observed that there were no gills in Leiopelma, respiration being
no doubt effected by this vascular tail. Noble questioned his statement and noted a
distinct suggestion of three branchial arches in an early intracapsular stage. However,
in the development of Leiopelmu, the respiratory mechanism appears t o be a8 follows.
Branches of the large omphalo-mesenteric veins ramify over the ventral abdominal
wall (Pl. 1,figs. 1 & 2),which has a considerable surface area since it is distended by
the yolk, There is little doubt that cutaneous respiration in this region is of significance. Furthermore, the tail of the intracapsular embryo, though not expanded, is
curved round and pressed against the inner wall of the capsule and its blood supply
shows that it is of importance as a respiratory organ. The posterior vena cava is
DEVELOPMENT 03’THE AMPHICOELOTJS FROGS
23
,established early in development as a large continuous vessel from the caudal vein to
the sinus venosus. Thus blood, which has reached the tail directly through the
-dorsal aorta and the caudal artery is carried back to the heart directly from the tail.
The later establishment of posterior cardinal veins does not interfere materially with
this direct venous channel from tail to heart, which is maintained until the tail is
reduced after hatching. Elsie M. & N. G. Stephenson (1947) found that well-developed
posterior cardinal veins as well as a posterior vena cava are present in the adult
Leiopelma and that a remarkable degree of variation occurs in the blood vessels of the
kidney region. The venous circulation in the embryonic and early post-hatching
stages of Leiopelma has no parallel amongst the Anura as far as I am aware, and the
development of postcaval and posterior cardinal veins has not yet been studied in
detail in either Leiopelma or Ascaphus. The postcaval vein is a formation which
appears for the first time in the vertebrate series in the Dipnoi and the direct
connection between the caudal vein and the posterior vena cava which ocours in the
development of Leiopelrraa is found also in Ceratodus.
Of other general features of the intracapsular embryo of Leiopelmu, the external
nostrils are situated a t the bases of slight ventro-lateral depressions. The eyes are
particularly prominent. bulging well out from the sides of the head. The mouth
aperture is ventrally placed some distance back from the anterior tip of the head.
An interesting feature is that the tongue is well developed from a fairly early intracapsular stage, whereas the published figures (Pusey, 1943) indicate that a tongue is
not developed in the tadpole of Ascaphus.
No larval teeth are developed in Leiopelm and there is no trace of teeth in a frog
which was sectioned four weeks after hatching. The yolk endoderm is proportionately
very large and there is still a quantity of yolk in the gut even four weeks after hatching.
It is quite clear that as yet the frog is not actively feeding, and that the late
development of teeth is correlated with this fact, Gastric glands, too, are only a t the
formative stage a t this time.
There is no coiling of the intestine as is a feature of tadpoles with vegetarian diet.
‘The fore gut of the intracapsular embryo is a straight tube and merely forms a slight
S-loopbefore becoming continuous with the yolk endoderm. The gut pursuesa straight
course back from the yolk endoderm to open a t the tip of the anal papilla, a prominent
median structure projecting between the developing hind limbs. The relationships of
the yolk endoderm to the remainder of the gut are similar to those of other Amphibia
with large yolky eggs undergoing holoblastic segmentation and direct development.
Archey (1922) observed that the rudiments of the fore limbs in Leiopelma were
evident a t a much earlier stage than the hind limbs. Noble (1927), who examined
some of Archey’s original material, stated as one of several oharacters in which
Leiopelmu differs from all higher types which have direct development, that the front
legs develop earlier than the rear pair. I have not been able to confirm these statements ; in sections of early intracapsular embryos I found that the fore limbs and hind
limbs are a t the same stage of development throughout. At early stages, the rudiments
of the hind limbs are placed side by side, separated only by the anal papilla, and
.situated close to the mid-ventral line. In these stages, they would be difficult to
detect without sectioning for they are covered from above and further concealed by the
base of the tail, whereas the fore limbs project laterally from the body and are more
readily observed. Lynn (19k2) found that the fore and hind limbs of Eleutheroductylue
nubicola appeared simultaneously and grew steadily throughout the intracapsular
period. He found that the well-marked period of rapid hind-limb growth commonly
associated with anuran metamorphosis is not present and that the growth of the hind
limbs accompanies that of the body and continues until hatching, a t which time it
has levelled off when the adult proportions have been reached. I n Leiopelmu, a similar
steady growth of the fore and hind limbs occurs during intracapsular development, but
a t the time of hatching the hind limbs have not reached adult proportions. They do
ti0 gradually in the succeeding weeks, when the tail is being absorbed (Pl. 2).
Certain frogs in which a more or leas direct development occurs, or in which a
24
N. Q. STEPHENSON : OBSERVATIONS ON THE
tadpole stage has clearly been suppressed, are stated to shorten the larval stages in the
development of various systems, since these stages are no longer required, and to
acquire their adult characters a t an early date. There is no evidence of this in the
slow intracapsular development of Leiopelma. For example, when the so-called
‘ froglet ’ hatches, i t has a typical larval arterial circulation in that there are four
aortic arches. A similar larval condition is found in the excretory organs a t this time ;
Dr. E. A. Fraser recently examined my slides and found that the pronephros, far
from being degenerate or suppressed, is large and well developed up to a few weeks
after hatching.
The lungs, also, are not sufficiently developed to be functional until some weeks
after hatching, and cricoid and arytenoid cartilages, which are separate in their development, are not fully chondrified until this time. I n the adult frog, E. M. Stephenson
has found that the lungs are relatively small. Noble (1931),without knowledge of t h e
condition in Leiopelma, suggested that the simple structure of the lungs of the adult
Ascuphus is a reduction, and that cold water had been a n important factnr in
permitting this reduction. In Ascuphus, too, lungs appear to be absent during the
greater part of larval life.
The external features of the Ascuphus tadpole show striking adaptations to life in
mountain brooks (Noble, 1927). The mouth region is particularly modified (Pl. 3,
fig. 5 ) . There is an enormous ventrally placed sucker and the larval teeth, set in double
rows, are specially adapted to obtain a grip on the rocks to which the tadpole clings.
By moving the jaws, the tadpole is able to move over a rocky surface without losing its
grip, and folds in the suoker are so arranged as to permit the mouth t o be opened for
this purpose without seriously detracting from the suctorial nature of the apparatus.
Unlike those of the embryo of Leiopelma, the external nostrils are dorsally placed
and are exceptional amongst Amphibia in that they are produced into irregular funnels
(Pl. 3, fig. 6). When a strong current is directed against them, these structures
bend down and close, thus controlling the current of water passing through the
nostrils and out through the median spiracle. No other tadpole is known to be
equipped with such structures, and the tadpole takes in food particles, up to a size of
1 mm. through its external nares (Noble, 1927). There is no suggestion, however,
that this is the only method of feeding, for particles of vegetable matter, scraped off
the moss-covered rocks as the tadpole moves along while still maintaining a grip by
means of its sucker, would no doubt be ingested through the plouth in the normal way.
Noble (1927) states that the lateral line system is not visible in the tadpole of
Ascaphus and he regards this as one of a number of adaptive features related directly
to life in the mountain torrent. The photograph of the dorsal region of the head of the
tadpole (Pl. 3, fig. 6). however, shows that such organs are present and gives some
indication of their pattern. It is clear from her description, of the Ascuphus tadpole
that these structures were first observed by Gaige (1820)) who described them as
muciferous cr-ypts ’. No trace of lateral line organs has been found at any stage of
the development of Leiopelma.
SUMMARY
AND DISCIUSSION.
The possible signifioanoe of the developmental features of Leiopelma and Ascuphus
is best considered in relationship to their systematic position amongst frogs. Nobls’e
(1931) recognition of the primitiveness of these two genera has been upheld and
confirmed by subsequent investigators. For example :1. There are 9 presacral vertebrae in Leiopelm and Ascaphus, but not more than I)
in any other living frogs.
2. Leiopelma and Ascaphw agree with primitive urodeles in that the interdorsals and
interventrals remain cartilaginous throughout life. The vertebrae are thus
amphicoelous, but this condition is not found in any other living Anura.
3. I n the Anura, the reduction of true ribs has reached an extreme. Only Leiopelma,
Ascuphus and the Discoglossid frogs retain ribs in the adult, though Pipids have
DEVELOPMENT OF THE AMPHTCOELOUS FROGS
25
ribs while larvae {Noble,1931). I n the adult LeiopelA, there are at least two
pairs of true ribs, and sometimes indications of a third pair are visible.
4. Abdominal ribs were well developed in the extinct Branchiosaurs. Such ribs
appear in the myosepta of the M. rectus abdominis of Leiopelma, and traces
occur in Bombina (Goette). Similar pieces of cartilage have been described in
Necturus, but, according to Noble, in no urodele are they as well developed as in
Leiopelm.
5. Leiopelma and Ascuphus are more primitive than other Anura in that the adult
retains two tail-wagging musclea, the pyriformis and caudali-pubo-ischiotibialis, although the tail has disappeared. (Noble, 1931.)
6 . Posterior cardinal veins are present in the larvae of frogs and urodeles. I n the adult
Leiopelmu, Ascaphus and Bombina, as well a8 in some adult urodeles, both
posterior cardinal veins and the posterior vena cava occur together. The
posterior cardinal veins of the adult Leiopelma are very well developed.
The above are some of the more important primitive characters of Leiopelma and
Ascuphus and in many of them the two frogs show a close relationship to urodeles.
Furthermore, many morphological characters previously considered to separate the
Urodela from the Anura are now found not to apply to these primitive genera. Their
anatomical relationships lend no support to the opinions advanced by Wintrebert
(1922), Holmgren (1933, 1939), Save-Soderbergh (1934, 1936) and Herre (1935) and
recently upheld by Jarvik ( 1942)that Urodeles and Anurans,have arisen independently
of each other. For example, differences in the connections between the quadrate and
the neurocranium in Urodela and in Anura seemed to demonstrate a gap which is now
bridged by Asmphus (Pusey, 1938, 1943) and by Leiopelma. The separation of the
nasal capsules and the presence of an ethmoidal region of the cranial cavity in Leiopelma
provides a condition intermediate between the two orders. The course of thehyomandibular nerve in Leiopelma displays relationships more comparable to those pertaining
to Urodela than to Anura.
But it is not only in many details of its adult anatomy that Leiopelma resembles
urodeles ; there are also striking resemblances in its development. The similarity is
well shown by summarizing the developmental features and comparing them with those
of urodeles laying large yolky eggs that undergo lengthy intracapsular development
either in water or on land.
I n Leiopelma :
1. Eggs are large and unpigmented (approximately 5 mm. in capsular diameter).
2. Intracapsular development on land continues for about 6 weeks,
3. Eggs are laid in a cluster (2-8) under moss-covered stones and logs.
4. The eggs are brooded and are guarded by the male.
Ti. Intracapsular movements occur and the intracapsular embryo is capable of
vigorous rotation by muscular movement. A considerable accumulation of
liquid early appears about the embryo and continues to increase In amount,
distending the egg capsule until, just before hatching, the capsular diameter is
about 1 cm.
6. A gular fold is developed.
7. At the time of hatching the embryo measures 17-19 mm. (In Ascaphus, intracapsular development proceeds in water for a month and a t the time of hatching
the larva measures about 13.5 mm. total length.)
8. At the time of hatching there is a considerable amount of yolk in the intestine.
This yolk gradually diminishes in amount, but is still evident even one month
after hatching.
I). The mouth at the time of hatching is still ventrally placed. It gradually a&sumes
a more terminal position as the yolk disappears.
20
N. C I . STEPHENSON
: OBSERVATIONS
ON T H E
10. External gills are not developed but the tail, though muscular and not expanded,
is vascular, and conspicuous blood vessels extend over the ventral body wall.
At the time of hatching, the lungs are not well developed and it is clear that
some time lapses before pulmonary respiration becomes effective.
11. Internal gills are not developed, but visceral clefts open into a deep braiichial
groove between the gular fold in front and the ventral body wall behind. I n
the intracapsular embryo, four clefts on each side open into this groove. After
hatching, the clefts, fold and groove disappear.
11’. Hatching occurs in late spring and in the subsequent weeks the tail is gradually
reduced. It has usually disappeared a t the end of a month. With the lose of
the tail, the reduction of yolk, and the change in mouth position, the tailed
froglet assumes the adult form.
I n Cryptobranchus a22egheniensis (Smith, 1912), a primitive urodele which lays its
eggs in water :-
I. Eggs are large and unpigmented.
2. Intracapsular development continues for six weeks before hatching occurs.
3. Eggs are fastened together like a string of beads.
4. Brooding habits are exhibited and the male guards the eggs.
5 . Tntracapsular movements occur a t first through ciliation (before the tail is
developed) and later by muscular movements.
ti. A gular fold is developed.
7 . The newly hatched larva measures about 43-25 mm.
8 . At the time of hatching the embryo retains sufficient yolk to last it for several
months. So far as yolk-retention is concerned, Smith (1912)regards the young
Cryptobranchus as an embryo rather than a larva.
!). The mouth a t the time of hatching is still ventrally situated; it gradually assumes
a terminal position as the yolk disappears.
10. Conspicuous blood vessels extend over the yolk sac of the newly hatched larva.
Respiration is not only by external gills, but also by capillaries lying close to
the surface all over the body. The tail may be of special importance as a
respiratory organ.
11. Internal gills are not developed, as is the case in all urodeles, but three gill openings
are present in the larva.
11’. Metamorphosis takes place a t the end ofthe second-year.
In the American Dusky Salamander, Dtmnognathus fuscus (Wilder, 1913. 1917), a
species which lays its eggs on land :1. Eggs are large, creamy white and unpigmented (3.5-4.0 mm. in diameter).
2. Intracapsular development (on land) continues for about 5 weeks.
3. Eggs are laid in a small batch of 20.
4. Brooding habits are exhibited, the eggs being guarded by the female.
5 . Intracapsular movements occur and after the 13th or 14th day the embryo begins
to make spontaneous movements.
6. A gular fold is developed.
7 . The newly hatched larva measures approximately 15 mm.
8. There is a considerable amount of yolk in the intestine a t the time of hatching.
9, A considerable accumulation of liquid early appears about the intracapsular
embryo and continues to increase in amount, distending the egg envelopes.
10. Apart from external gills, which develop as early as the 16th day, blood vessels
extend conspicuously over the yolk.
11. Internal gills are not developed, as in all urodeles, but two olefts open, the more
anterior being just posterior to the gular fold.
DEVELOPMENT OF THE AMPHICOELOUS FROGS
27
12. For approximately two weeks after hatching the larva remains terrestrial, but then
becomes aquatic. Thus, during the following autumn, winter and early spring
it remains in moving water which does not freeze over. Metamorphosis o c c m
in the late spring and involves :(a) Change in body proportions.
( b ) The median fin atrophies and the tail develops a more rounded contour.
(c) External gills disappear and the clefts close.
There ia also much similarity between the development of Leiopelma and that of the
.anuran Ekutherodacty~?uewhich has direct development.
I n Eleutherodactylw,(Sampson, 1904 ;Lynn, 1942 ; Gitlin, 194-4):1. Eggs are large and unpigmented.
2. Intracapsular development varies in different species, but in E. nubicola (Lynn)
occupies 26 days.
3. I n E. nubicola, 26-66 eggs are laid in a cluster.
4. I n E. nubicola, females with spent ovaries and enlarged oviducts remain with the
eggs.
5. Rotation of the embryo of E. portoricenaie has been described by Gitlin.
6. Dermal folds in gular region, which disappear before hatching, are described.
7 . E. mrtinicensis measures 23 mm. a t hatching (Sampson).
8. I n this species also, Sampson states that the yolk holds the same relationships to
the digestive tract as it does in other yolk-laden amphibian eggs such as those of
urodeles and caecilians. There is a considerable amount of yolk present at the
time of hatching, but the frog may start to feed before this yolk is entirely
absorbed.
9. Usually external gills are not developed, but transitory external gills are stated by
Noble (1927) to be present in two West Indian species.
10. Internal gills are not developed.
11. Visceral pouches appear in intracapsular development, but these never become
open clefts.
12. The tail is characterized by the fact that it is very flat and leaf-like and is clearly a
vascular organ with respiratory function.
The above indicates many points of similarity between the development of urodeles
and that of frogs in which there is direct development. Urodeles in general have a
fairly direct development and little transformation is required to change the larva into
an adult. With frogs, development is typically indirect and the free living tadpole may
be extremely specialized in adaptation to its environment. A natural correlative is
this: the greater the degree of larval specialization in the tadpole, the more striking is
the anuran metamorphosis. Ascaphue truei, with its large eggs, lengthy period of
intracapsular development in water, followed by a free-living mountain brook tadpole
stage, seems t o bridge the gap between direct and indirect development among
Amphibia.
ACKNOWLEDGMENTS.
This paper has been prepared in the Department of Embryology, University
College, London. I am most grateful for the advice and helpful criticism of Professor
G . R. de Beer, F.R.S., under whose supervision I have worked. I would also like to
take this opportunity of thanking Mr. F. J. Pittock, F.R.P.S., A.L.S., for photographs
and Mr. H. E. Barker for assistance in the preparation of materials.
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Development of the ttmphicoelous frogs.
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