FATE OP THE BLA.STOPORB IN RANA. TEMPORARIA.
49
Note on the Pate of the Blastopore in R ana
temporaria.
By
Harold Sidettotbam, M.R.C.S.
With Plate V.
IN the description given in Balfour's ' Comparative Embryology' of the development of the Amphibia, that portion
of it which specially relates to the anourous Amphibia is
mainly founded on the researches of Gotte on Bombinator. I t
is there stated that 1 "at first the mesenteron freely communicates with the exterior by the opening of the blastopore. The
lips of the blastopore gradually approximate, and form a passage, on the dorsal side of which the neural tube opens . . . .
The external opening of this passage finally becomes obliterated, and the passage itself is left as a narrow diverticulum,
leading from the hind end of the mesenteron. into the neural
canal. It forms the post-anal gut and gradually narrows, and
finally atrophies. At its front border, on the ventral side,
there may be seen a slight ventrally directed diverticulum of
the alimentary tract, which first becomes visible at a somewhat
earlier stage. This diverticulum becomes longer and meets an
invagination of the skin, which arises in R a n a t e m p o r a r i a
at a somewhat earlier period than represented by Gotte for
Bombinator. This epiblastic invagination is the proctodaeum,
and an anal perforation eventually appears at its upper
extremity."
1
Balfour, 'Comp. Embryology,' 2nd edit., vol. ii, p. 130.
VOL. XXIX, PART 1 .
NEW SEB.
D
50
HAEOLD SIDEBOTHAM.
1
In a paper " O n some Points in the Early Development of
R a n a t e m p o r a r i a " Professor Spencer states that he is unable
to find any trace of the inclusion of the blastopore by the
neural folds, or any trace of the closure of the blastopore itself.
He also states that soon after the stage has been reached in
which the neural folds have met, the hinder part of the neural
tube becomes closed, though the blastopore itself remains
open.
Miss Johnson and Miss Sheldon have also published some
" N o t e s on the Development of the Newt," 2 in which they
make some remarks with regard to the Frog. They apparently
agree with Professor Spencer in the opinion that the blastopore
persists as the permanent anus.
Mr. Herbert Durham 3 has also stated that he fully agrees
with Professor Spencer in regard to the fate of the blastopore.
Being interested in this question I have cut a large number of
series of sections of R a n a t e m p o r a r i a . A careful study of
these sections leads me to come forward and express my
opinion, and that with some degree of confidence, as more than
sixty embryos have been examined, that the history of the
blastopore more resembles the account of it given by Balfour
than that given by any of the other authors above alluded to.
After the formation of the neural folds, and while they are'
still widely separated, the mesenteron opens to the exterior by
means of the blastopore, which is situated at the extreme
posterior end of the embryo. Fig. 1 shows a median, vertical,
longitudinal section, taken at this stage; in it can be seen a
well-marked diverticulum from the hind end of the mesenteron, dipping down towards a distinct pit in the epiblast
below the blastopore and quite separate from it. This is the
rectum forming, and advancing to meet a true proctodaeal pit.
This embryo has three mesoblastic somites. As the neural
folds grow up to meet each other they do not enclose the blastopore, but reach as far as its dorsal rim.
1
2
3
'Quart. Journ. Micr. Sci.,' vol. xxv, Supplement, IS85, p. 123.
'Quart. Journ. Micr. Sci.,' vol. xxvi, 1886, p. 573.
'Quart. Journ. Micr. Sci./ vol. xxvi, 1880, p. 508.
PATE OF THE BLASTOPORE IN RANA TBMPORARIA.
51
Fig. 2 shows the next stage in an embryo with five mesoblastic somites. The neural folds have met and form a tube,
bending over the posterior end of the embryo, and opening
through the dorsal region of the blastopore, which has become
narrower and longer. Below the blastopore the proctodseal
invagination may be seen to have increased considerably in
depth as also has the rectal diverticulum from the mesenteron.
In fig. 3 the same parts can be recognised, but the closure of
the blastopore has proceeded further. This stage has six mesoblastic somites. The septum dividing the rectal diverticulum
from the proctodseal invagination has become perforate. The
section of which fig. 3 is a drawing is not quite in the right
plane for showing the neurenteric canal.
Fig. 4 shows the next stage in an embryo with eight mcsoblastic somites. The blastopore is completely occluded, and is
represented by a column of epiblastic cells (ep.) which may
be recognised by their containing much pigment. The anus
communicates freely with the alimentary canal, and the
neurenteric canal is still well marked.
There is a slight
dilatation of this canal at the junction of its dorsal and ventral
limbs, which dilatation is the post-anal vesicle.
In the next stage, with nine mesoblastic somites, represented
by fig. 5, the tail has just commenced to bud out. The remnant of the blastopore is represented only by a little heaping
together of the epiblast cells, where the blastopore opened to
the surface. The neurenteric canal has become occluded, but
still can be traced up as a solid rod of cells, representing the
post-anal gut, commencing from the normal position. There
is a diverticulum from the proctodseum just before it opens to
the surface which is the rudiment of the allantoic bladder.
Thus the history differs from that given by Balfour, in that
the neural folds do not enclose the blastopore, the closure of
the blastopore being eflFected subsequently to the meeting of
the neural folds. My conclusion differs essentially from the
description given by Professor Spencer, inasmuch as I find
that the anus is not derived from a persistent blastopore, but
is formed from an independent proctodseal invagination.
52
HAROLD SIDEBOTHAM.
Miss Johnson and Miss Sheldon, writing with reference to
the Newt, 1 incline to the conclusion that the tail, as well as
the post-anal gut, is a secondary structure developed after the
permanent anus. Of course this view would be equally applicable to the same structures in the Frog, if Professor
Spencer is correct as to the fate of the blastopore.
But I think that the condition which obtains in the Frog at
the stage when the blastopore is still just open represents
an extremely primitive condition, for Balfour, in his paragraph
with reference to the post-anal gut and neurenteric canal,2
comes to the conclusion that the neural and alimentary canals
must have had a common opening, probably into a dilated
vesicle, before going directly to the exterior. This is exactly
what is found in the Prog just before the rectal diverticulum
becomes perforate.
Professor Spencer only figures one longitudinal section, and
this is from a stage subsequent to that in which the closure of
the neurenteric canal takes place, a stage, moreover, at which
all trace of the blastopore has gone.
During the time the blastopore is open it always runs in a
line with the mesenteron and opens posteriorly, while the rectal
diverticulum always runs ventrally, and keeps at about the
same angle to the mesenteron, even after the tail has become
fairly well developed.
In Professor Spencer's fig. 15 the canal, the extremity of
which he marks (an.) and describes as the blastopore, I think
must be the rectal diverticulum, as I find just the same condition as there represented in my sections taken from embryos
at a corresponding age.
His figs. 5 and 6 are taken from an earlier stage than his
fig. 15, and show just the same features as do mine, but the
appearances they present may easily be interpreted in accordance with my view; the opening he marks bl. in both figures being
the proctodseal invagination. In his fig. 5 the blastopore is
1
Loc. cit.
2
' Comp. Embryology,' vol. ii, cliap. xii.
FATE OP THE BLASTOPORE IN RA.NA TEMPORAMA.
53
continued posteriorly from the point ne. My figs. 6 and 7 also
show this.
Mr. Durham's drawings also are all taken from a, later stage,
but are not intended to illustrate the history of the blastopore
but the fact of there being a neurenteric canal. In the earliest
stage •which he figures, the rectum is the ventrally inclined
diverticnlum, and is shown exactly as I find it. The blastopore is occluded previously to this stage.
Only one series out of over sixty has failed to show this
history. It was abnormal in other respects than the blastopore, and was probably pathological altogether.
In conclusion, I wish to express my sincere thanks to Professor Milnes Marshall for looking at my specimens, and
also for much kind advice in reference to the subject of this
paper.
54
HAUOLb S1DEBOTHAM.
DESCRIPTION OF PLATE V,
Illustrating Mr. Harold Sidebotham's "Note on the Fate of
the Blastopore in Rana temporaria."
Reference Letters.
ai. Alimentary canal, all. Allantois. hi. Blastopore. ep. Epiblast. m.
Mesoblast. n. c. Neurenteric canal, not. Notochord. n. g. Neural groove.
p. a. g. Post-anal gut. pr. Prootodceum. n. c. Neural canal, r. d. Rectal
diverticulum. sp. c. Spinal cord. x. Yolk.
FIGS. 1—5.—Median vertical lougitudinal sections.
Fig. 1. From an embryo witb three mesoblastic somites. The neural
groove is still open, except at the anterior end; the blastopore is widely open;
and the rectal diverticulum well marked.
Fig. 2. From an embryo with five mesoblastic somites.
Fig. 3. From an embryo with six mesoblastic somites.
Fig. 4. From an embryo with seven mesoblastic somites.
Fig. 5. From an embryo with eight to nine mesoblastic somites.
FIGS. 6 and 7.—Transverse sections from an embryo slightly older than
Fig. 1.
Fig. 6. Taken along line a—a in Fig. 1.
Fig. 7. Taken along line b—h in Fig. 1.
Fig. 6.
p.a,.Q.