AMER. ZOOL., 15:679-689 (1975)
Morphology and Genetics of Sea Urchin Development
RALPH T . HINEGARDNER
Division of Natural Sciences, University of California, Santa Cruz, California 95064
SYNOPSIS. Sea urchins can be raised from egg to egg in the laboratory. With proper food, the
larvae can be grown to maturity in about 3 weeks. When mature larvae are exposed to the
proper chemical cues metamorphosis occurs. Over the next 5 days the small urchins develop
internal organs and then begin to feed. Sexual maturity can be reached in as little as 4.5
months. By then the urchin is about a centimeter in diameter.
Several different approaches to the study of developmental genetics are covered. These
include: (i) hybrids between the sand dollars Dendraster and Encope, in which both crosses
produce offspring that have predominantly paternal characteristics; (ii) a preliminary
description of two mutants, one which produces abnormally shaped blastula that may lead to
a significant number of exogastrulae, and another that produces a large number of fourpart symmetrical urchins; (iii) urchins produced by parthenogenetic activation and from
reaggregated larval cells.
INTRODUCTION
The first reasonably accurate description
of sea urchin early development was written by M. Derbes, in 1847. He described the
embryological development of Echinus esculentus. On the whole, Derbes' description
was fairly complete. He deduced the presence of the jelly coat, described formation
of the fertilization membrane, and recognized that the mature egg arose from an
earlier germinal vesical stage. He pointed
out that it was possible to identify the sexes
by the appearance of the gonads, and he
added that the female gonads "tasted more
agreeable and more pleasurable."
Some of Derbes' observations were not
totally accurate. Had they been, the history
of biology might have been different. Because he could not see the male nucleus in
the fertilized egg, he concluded that the
sperm did not contribute to the embryo but
only activated the egg. His conclusions were
in keeping with the beliefs of his time. Ironically, 30 years later, the main piece of evidence Hertwig (1876) used to show that the
male gamete did contribute to the embryo
was the fact that the male nucleus could be
seen fusing with the female nucleus in sea
This research was supported by a National Science
Foundation Grant. I wish to thank Margaret Swanson
and Kathryn Boyer for their conscientious assistance.
urchin eggs.
Derbes was misled in a few other places.
He concluded that the blastopore became
the mouth. This is a mistake that is easy to
make unless development is observed carefully. He attempted to follow the complete
life cycle and he thought that the starved
pluteus represented further development
toward the urchin and that the little ciliated
blob the pluteus finally degenerates into
after many days without food was a particularly critical state in the urchin's life cycle.
Subsequent observations corrected Derbes' mistakes, and all that he attempted to
do has since been accomplished. It wasn't
until the last decades of the 19th century,
with the work of Hertwig, Boveri, Herbst,
Driesch and others, that the sea urchin egg
began to play a large role in the study of
development. Since then something like
3,000 papers have been published that in
one way or another are concerned with the
sea urchin embryo.
Around the turn of the century, a
number of biologists succeeded in raising
larvae to maturity. Usually this was done by
frequently changing the sea water the cultures were growing in, the food source
being the planktonic algae that came in with
the water. Growth was slow, but judging by
published illustrations, it was normal. Some
of these larvae were taken through
metamorphosis. Bury (1895) was one of the
679
680
RALPH T. HINEGARDNER
first to accomplish this. Development was epaulets form. Three pedicellariae appear,
seldom carried further. The most complete two on the right side and one at the posdescription of larval development that has terior end of the larvae. These will later be
been published is still that of MacBride incorporated into the anatomy of the ur(1903). By the 1920's the basic features and chin after metamorphosis. Figure 1 illusmany of the details of the sea urchin life trates the anatomy of a mature larva. Larvae stop growing when they are mature,
cycle were well understood.
and once they reach this size they are competent to metamorphose. If metamorNORMAL DEVELOPMENT
phosis does not occur the larvae will conAt various times, attempts have been tinue to feed, though at a much lower rate,
made to raise urchins as laboratory animals. and can be kept for several more months.
In retrospect, it isn't clear why these were However, they are able to metamorphose
not successful. As it turns out, the for a period of only a few weeks. After that,
technique is not difficult once the proper they slowly begin to degenerate, and finally
procedures are worked out. One problem end up as ciliated spheres.
During larval growth, the embryonic urmay have been that the larvae and young
urchins were being given food organisms chin is also developing. On about the 7th
they do not readily eat. For example, Har- day after fertilization, it begins to form out
vey (1949) tried to raise Arbacia larvae on of the union of a small portion of the ecdiatoms. These are a poor food source for toderm on the left surface of the larva and
all species I have tried to raise, including the middle left hydrocoel, which arose
Arbacia. When the right food organism is from the coelmic pouches formed after gasused along with the proper technique, it is trulation. The developing urchin, while it is
not difficult to raise urchins from egg to egg in the larva, is called the rudiment. The larva
in the laboratory (Hinegardner, 1969).
The basic features of normal Lytechinus
pictus development have been described
(Hinegardner, 1969). At 18°C larvae grow
in the laboratory from 0.3 mm long plutei
to 1.5 mm mature larvae in approximately
3 weeks. Arbacia and several other species
grow at about the same rate. Earlier we kept
our animals at 24°C; however 18° is somewhat better. Growth rate is dependent
upon temperature and several factors. It is
decreased if the larvae are crowded. We
routinely provide about 10 ml of sea water
per larva, and completely change the
water after 1.5 weeks. If too little food is
provided, growth can be extended over
several months. We use an unidentified
species of Rhodomonas for food and give our
cultures about as much as they will consume
in 24 hr.
Developmental morphology
The external features of the developing
larvae gradually become more complex as
the larvae grow. New spicules and their as- FIG. 1. A mature larva of the sea urchin Lytechinus
sociated arms arise, and in some species, pictus. p, Pedicillaria; r, urchin rudiment; s, larval
such as Lytechinus, dense ciliary bands called stomach.
SEA URCHIN DEVELOPMENT
681
serves as a source of nutrient and protec- weight organic compound(s) that is formed
tion for the growing rudiment. The rudi- by bacteria. A solid surface greatly faciliment is not a little urchin, but only a portion tates metamorphosis, though larva will
of the developing ventral half of the urchin, sometimes metamorphose while they are
and consists primarily of ventral skeleton held on a suction pipette or by forcepts.
and water vascular system. The rest of the The nature of the chemical cue and a more
ventral half, as well as almost all the dorsal detailed description of the metamorphic
and internal structures, develop sub- process are described in Cameron and
sequent to metamorphosis. Though most Hinegardner (1974). In an hour or less
of the larval biomass ends up in the urchin, after the initial stimulus, the major external
it does so after passing through a period changes from larva to urchin have taken
during early metamorphosis when many of place and by 24 hr the individual looks like
the cells are broken down and the larval a little urchin. There are still major internal
material is little more than a lump of pro- rearrangements that take 5 or 6 more days
toplasm on the top of the newly metamor- before the urchin begins to feed. These
phosed urchin. The urchin develops its include formation of a complete gut along
own mouth, anus, and most of its internal with mouth, anus and teeth, and the dorsal
organs.
skeleton. When these internal changes are
complete,
the urchin begins to feed.
The nemertines and insects have somewhat similar development. In both these
there are imaginal discs which give rise to Urchin growth
portions of the adult. Some authors have
In the laboratory, we feed our young
also called the urchin rudiment an imaginal
disc. However, it is not really the same animals a surface-adhering diatom belongthing. Unlike imaginal discs, the rudiment ing to the genusNitzschia, which is grown on
shows the beginnings of differentiation plastic dishes. The urchins are transferred
from its first appearance, and as it grows, to fresh dishes about every 5 days, or when
the tube feet and spines are clearly visible. they have consumed most of the algae.
When the urchin reaches a diameter of 9
By metamorphosis it is well differentiated.
to 10 mm it can be induced to spawn. In our
laboratory the males of Lytechinus mature
Metamorphosis
earlier than the females and can spawn at 9
Once the larva is mature, it has to pass mm. The females usually do not spawn
successfully through metamorphosis and until they are 10 mm or larger. The entire
then grow to a sexually mature adult. life cycle from egg to egg can take as little as
Metamorphosis is not obligatory, and if the 4.5 months in the laboratory if the urchins
mature larva is not exposed to the right are well cared for. With more ordinary
cues, it never becomes an urchin, and in- care, 6 months is the more usual maturing
stead, as I have already mentioned, it even- age. Table 1 outlines the time course of the
tually degenerates into a ciliated sphere developmental process.
On the whole, sea urchins are not apprecthat finally dies. This is not necessarily true
for all echinoderms. The larva of the sea iably more difficult to raise than other
star Mediaster aequalis, if it is not exposed to
the tubes of the polychaete worm on which TABLE 1. Development of Lytechinus pictus at 18°C.
it normally settles, can live for up to 14
0
months and still remain capable of Fertilization
Begins to feed
2 days
metamorphosis (Birkeland et al., 1971).
Larva matures
3 weeks
5 days after
If the larvae of Lytechinus, Arbacia, or Urchin begins to feed
metamorphosis
many other species of sea urchins are ex- Sexual maturity
4.5 to 6 months
posed to the appropriate cues, they begin Life span
At least 7 years in the
laboratory; 3-year
metamorphosis within a few minutes. The
average in the wild
important cue for all of these species turns
(Ebert, 1975).
out to be an unidentified low molecular
682
RALPH T. HINEGARDNER
laboratory animals. The inconveniences
that do exist almost all stem from the fact
that urchins live in water and we do not.
Consequently, there is a lot of water handling. Aside from this the animals are almost as hardy as mice or Drosophila and
present no particular problems.
LABORATORY MAINTENANCE
The previous sections of this paper cover
the essential features of laboratory culture.
In order to utilize individuals in genetic
studies mortality has to be kept low once the
animals have matured. This section will describe the methods we use for maintaining
animals over long periods and in good
health. "
At present we have about 400 adult urchins averaging about 2 cm in diameter.
Most of these are laboratory raised and are
distributed among eight 20-gallon tanks.
The tanks are kept at a temperature of
about 15°C either in a cold room or by refrigeration. Each tank is aerated and has its
own external water filter which contains a
layer each of Dacron wool and crushed
dolomite. About 4 liters of sea water are
removed and replaced with fresh sea water
once a week for each tank. Food is almost
solely the giant kelp Macrocystis. This
species is used primarily because it is convenient. Several other species of algae, such
as Egregia, Laminaria or Viva can also be
used. The urchins are fed as much algae as
they will consume. With this feeding they
can be induced to spawn about once a
month.
Spawning
Many of our urchins have been made to
spawn repeatedly, sometimes as often as
once every 2 months for more than a year.
Those carrying unique developmental
characteristics have been particularly well
used. For these, as well as others, we take
particular care not to kill the source of our
eggs. After trying various spawning procedures, we have settled on injection of 0.5 M
KC1 as our routine method. Acethylcholine
(Hinegardner, 1967) can also be used but is
less convenient. The animals are never al-
lowed to remain out of the water for more
than a minute. KC1 itself is not harmful if
used in moderate amounts; however
Lytechinus is very sensitive to drying and
though an individual may appear normal
for several days after it has been left out of
the water, it soon begins to lose spines and
dies within a week or so. To spawn our
animals we inject them only with enough
KC1 to induce spawning, which is seldom
more than 0.5 ml for a large urchin, and
usually closer to 0.25 ml. We immediately
place the urchin in a 500-ml beaker containing sea water from the tank the urchin was
living in, and allow the urchin to crawl
around freely. The animal almost invariably crawls up the side to the water surface
and eggs or sperm collect undisturbed on
the bottom. Broken spines and debris are
later removed by filtering the eggs through
HO-jitm nylon mesh. If sperm are to be
saved, they are drawn off the bottom with a
pipette and centrifuged at 800 g for 10 min.
The sea .water is removed and the concentrated sperm pellet stored on ice or in the
refrigerator.
Before fertilization, the eggs are washed
several times in fresh sea water to remove
any fertilization inhibitors that come from
the adult animals. From this point on, the
eggs and sperm are handled under standard procedures such as those described by
Harvey (1956), Costello et al. (1957), Tyler
and Tyler (1966), and Hinegardner (1967).
DISEASES
It is surprising that in the more than 5
years we have been raising animals in closed
laboratory systems, we have had no serious
outbreak of any disease. This is in spite of
the fact that all the food for the adults
comes from the ocean and that we take no
particular precaution to keep out potential
disease organisms. The only outbreak of
any kind that we have had in some of our
tanks was an infection caused by an
amoeboid flagellated protozoa that grew in
dense patches on the urchins. This caused
massive congregation of echinochromecarrying cells in the infected area which
made the patches bright red. Though these
patches became necrotic and spines fell off,
SEA URCHIN DEVELOPMENT
in time most of the urchins cured themselves and we have since had no trouble.
All this suggests that Lytechinus, at least, is
remarkably resistant to disease. This makes
their culture relatively easy. The whole area
of disease resistance is an aspect of
echinoderm biology that deserves more attention than it has been given. The only
observation we have made so far that relates to sea urchin defenses against disease
is the frequent appearance of the echinochrome-carrying cells in areas of infection,
or all over animals that are obviously not
healthy. What role they actually play is not
clear.
683
this subject which has been reviewed many
times. The following are a few of the books
covering this subject: Davidson (1968),
Giudice (1973), and Czihak (1975).
Hybrids
GENETICS
In contrast to this extensive literature on
the timing and overall pattern of gene action, the literature on single gene effects
and their timing is much sparser. Most of
the work comes from studies of hybrids,
though the techniques of molecular biology
are also beginning to yield conclusive results. Only the use of hybrids will be considered here.
Barrett and Angelo (1969) used hybrids
Almost all of sea urchin genetics has been
limited either to studies of inter-specific
and inter-generic hybrids or to the area of
molecular biology. To some extent, hybrid
studies have been forced on the sea urchin
embryologist because genetics at a more
refined level has not been possible. Hybrid
studies have been useful and they played a
particularly important part in the early investigation of the role of the nucleus vs. that
of the cytoplasm. Horstadius (1973) presents an extensive discussion of these early
investigations. In many ways, research at
the molecular level is just beginning, in
spite of the fact that the literature is already
very extensive.
The general features of echinoderm development that have emerged so far from
the use of both hybrids and molecular biology are these: During oogenesis, messenger RN A is synthesized and transported
to the cytoplasm in an inactive form. Upon
fertilization, or shortly afterward, this RNA
begins to participate in protein synthesis. In
general, most of the proteins synthesized
up to mesenchyme blastula are translated
from this RNA. The early embryo, therefore, bears primarily maternal characteristics. Shortly before gastrulation, the proteins synthesized on RNA from the embryonic genome, which consists of both
maternal and paternal chromosomes, begins to play a role, and from then on the
embryo bears characteristics of the combined genomes. There is a vast literature on
franciscanus and demonstrated that hatching enzyme has characteristics derived
from the maternal genome. At the prism
stage, the enzyme alkaline phosphatase
produced by hybrids between S. purpuratus
(female) and the sand dollar Dendraster excentricus (male) has activity intermediate
between the two species (Flickinger, 1957).
The same holds for the enzyme aryl sulfatase from a cross between Allocentrotus
fragilis and S. purpuratus. Electrophoretic
mobility of the enzyme was also intermediate (Fedecka-Bruner et al., 1971). In
general, hybrids show characteristics of the
maternal parent prior to gastrulation.
After gastrulation their characteristics are
intermediate between the parents. This is
in agreement with the evidence from
molecular biology.
Another form of hybridization is the use
of eggs that have had their nucleus removed. These are called merogones.
Merogones can be formed either by centrifugation into halves, using the anucleate
half, or a portion of the cytoplasm and the
nucleus can be cut off prior to fertilization.
Some eggs are particularly well suited to the
latter procedure since the nucleus lies close
to the cell membrane prior to fertilization. '
The subsequent larvae, if they are viable,
usually have characteristics of the male
species.
This technique played a key role in early
research demonstrating that the nucleus
carried the hereditary characteristics of the
between Strongylocentrotus purpuratus and S.
684
RALPH T. HINEGARDNER
organism. Horstadius (1936) used it in a Dendraster (female) x Encope (male) had
particularly sophisticated way to dem- only red pigment. Pigment cells in the reonstrate the role of the micromeres. He ciprocal cross were black, though the cells
formed a heterospecific merogone using were not as elongate as those in pure DenParacentrotus cytoplasm and Psammechinus draster. There were only about 1% red
sperm. At the 16-cell stage, the micromeres spherical cells.
were removed and fused to a normal
This same paternal dominance also exParacentrotus embryo that had previously tends to the amount of the enzyme /3-1,
had its micromeres removed. The skeleton 3-glucanase that is synthesized by the plutei
and general shape of the pluteus had the (Vacquier, personal communication).
appearance of a Psammechinus pluteus, even Plutei carrying the Encope sperm genome
though the nuclei of the primary mesen- produce more. However, the time at which
chyme cells were all that had a Psammechinus synthesis begins is determined by the egg.
genome. The reciprocal cross gave recip- The same general features of paternal
rocal results.
dominance that have been described here
Many hybrids fail to develop much also apply to crosses between Dendraster and
beyond blastula. An example is the cross Encope grandis. Again, the male genome is
between Paracentrotus lividus (female) and dominant.
Arbacia lixula (male) which was extensively
At first glance, at least, these results are
studied by Whiteley and Baltzer (1958). hard to explain. They suggests a more
There are many other papers on this sub- complex control of gene activity than curject dating from the last century to the pres- rent theories of gene control provide for.
ent. In fact the first review of sea urchin One of the odd features of these crosses is
hybridization is by Tennent in 1910. More that the Encope x Dendraster cross is not able
recent reviews are in Harvey (1956), to hatch from its jelly coat. They have to be
Giudice (1973), and Horstadius (1973).
hatched artificially with the aid of proteolyAlmost no hybrids have been raised tic enzymes.
Unfortunately, we were never able to folbeyond plutei. With the development of
procedures for laboratory culture, this is low our crosses through to maturity. The
now possible, and one cross between two incubation system failed on a hot day and
different genera can grow at least to young the young sand dollars died. At the time,
adults. This cross is between the sand dol- both crosses were feeding and growing, allars Dentraster excentricus and Encope califorthough not as well as the parental types.
nicus (Hinegardner and Vacquier, unpub- Even this accident yielded some informalished). Figure 2 illustrates the appearance tion. Encope comes from the warm waters of
of the mature larvae and the young urchins Baja California; Dendraster from the
produced by the various possible crosses. California coast. The inside temperature in
The most striking feature is the dominance our incubator reached about 35°C for a
of paternal characteristics in both hybrids. period of several hours. Only the Encope x
The shape of the mature larva is close to Encope adults survived. The Dendraster
(female) x Encope (male) individuals did
that of the paternal species.
Pigmentation is also paternal. The young not, indicating that they were not totally
Dendraster sand dollars have elongate black Encope in all their characteristics.
pigment cells, particularly in their ventral
epithelium, and very little red pigment. In Classical genetics
contrast, young Encope at the same stage
have bright red spherical pigment cells and
All the foregoing examples of sea urchin
little or no black pigment. The hybrids, genetics used hybrids. These can tell a lot,
FIG. 2. Mature larvae and one-day-old urchins from for each cross. The Dendraster x Encope larva pictured
normal and hybrid crosses between Dendraster excen- here has somewhat longer arms than the typical progtricus (D) and Encope californicus (E). A, D x D; B, Ex
eny of that cross.
E; C, D x E; D, E x D. The female parent is given first
SEA URCHIN DEVELOPMENT
685
686
RALPH T. HINEGARDNER
but there is little room for manipulation or
experimentation, and it is not possible to
examine the effects of selected genes in
homozygotic conditions. Hybrids are the
ultimate in hererozygosity. The literature
on sea urchin genetics using classical procedures such as back crossing and inbreeding is non-existent. This situation has
begun to change. In our laboratory we now
have a number of inbred lines of the sea
urchin Lytechinus pictus. Many of our animals are descendants from larvae of crosses
that yielded high instances of developmental abnormalities. Two of these inherited
abnormalities will be described here. We
have not had enough time to do the crosses
necessary to define their exact genetic basis.
Square. This is the term we have used to
designate urchins that are four-part, rather
than the usual five-part, symmetrical. Figure 3 is a photograph of one of these urchins. This is an abnormality that shows up
infrequently in out crosses, but in one of
our inbred lines between 1 and 10% (depending on the particular parents used) of
the progeny will be square. Actually square
is only one consequence of this inheritance.
The urchins can be two-, three-, four-, fiveand sometimes six-part symmetrical. In
other words, there seems to be a loss of
symmetry control. However, only the four-,
five- and six-part urchins have ever been
able to develop. Four is the common abnormality, thus the designation—square.
Square is apparently controlled by more
than one gene, since in only one out of six
crosses between unrelated square urchins
were a significant number of square offspring produced. In that cross about onethird of the Fi urchins were less than 5-part
symmetrical. Further analysis has to wait
until our inbred urchins reach maturity.
Exogastrula. This is a maternal effect and
the number of abnormal embryos that are
produced is unaffected by the male used to
fertilize the eggs. Like square, we have not
yet had sufficient time to determine the
genetic basis of this characteristic. We do
know that it is not temperature sensitive.
At present we have one female from an
inbred line that consistently produces embryos with some degree of this characteristic. Usually less than 1% of the embryos
exogastrulate. The more prevalent effect is
a condition earlier in development which
leads to the formation of embryos that are
distinctly oval at the start of gastrulation.
Most of the embryos have that appearance;
those that exogastrulate may be the ones
that do not recover from their abnormal
shape. Figure AA illustrates a normal early
gastrula and 4B, a normally gastrulating,
but oval, embryo produced by this female
urchin. Figure 4C is a slightly older embryo
that was beginning to exogastrulate, and
4D, a still older embryo. Figure 4£ shows a
pluteus with a completely everted gut. The
latter is the extreme condition. Others may
only have partially protruding guts and in a
few the guts barely protruded, with the
mouth end therefore not quite reaching the
stomadeal opening. As Horstadius (1949)
has shown, the stomadeal opening forms
whether or not the gut is there. As with LiCl
treatment, which also induces exogastrulation, the gut is able to differentiate into a
tripartite structure even when it is wrong
side out and in an abnormal position.
EXPERIMENTAL TECHNIQUES
FIG. 3. Four-part symmetrical Lytechinus pictus.
Animal is about 2 cm in diameter.
Sea urchin eggs can be manipulated by a
FIG. 4. A, Normal early gastrula of Lytechinus pictus. note the elongate shape. C and D, Two stages in
B, Gastrulating embryo from exogastrula mutant; exogastrulation. E, Exogastrulated pluteus.
688
RALPH T. HINEGARDNER
large number of different experimental
techniques. Many of these are simple
enough to be used as routine procedures.
Twinning
Lytechinus embryos can be twinned by
separating the blastomeres at the two-cell
stage. With the methods we now have,
about 50% of the twin pairs will develop to
mature larvae. We have raised some of
these to mature, sexually productive adults.
The mature larvae as well as the young
urchins are the same size as normal animals
and behave normally in all respects.
Parthenogenesis
Unlike the eggs of most other animals
that are used in experimental embryology,
sea urchin eggs can be artificially activated,
and therefore, a homozygous population of
animals can, in principle, be produced.
Harvey (1956) has reviewed the earlier literature on urchin parthenogenesis. Until
recently, Lytechinus pictus was an exception,
and as far as I know no one had been able to
induce parthenogenetic development. We
have been able to obtain parthenogenesis in
this species and the general procedure and
some of the early results will be published
elsewhere (Brandriff et al., 1975). Of approximately 30 females we have tried, only
three consistently produce eggs that can
both be activated and raised to maturity. Even with these females, only about one
out of 10 million eggs can be raised to a
feeding urchin, and all of these are at leasta
little odd. The tube feet may be shorter
than normal, the test malformed, or the
genital openings present in the wrong
places. All are stupid, even by sea urchin
standards, and require special care. The
male is supposed to be the digametic sex in
urchins, and older parthenogenetic urchins that have died have all been females,
as would be expected.
Reaggregation
In 1962, Giudice discovered that the cells
of early sea urchin embryos could be disaggregated, then reaggregated, and that
reasonably normal plutei would reform.
FIG. 5. Three Arbaciapunctulata urchins grown from
reaggregated cells from 16-cell embryos.
He used Paracentrotus lividus, but a number
of other species can also be used. This discovery is of particular importance, since at
the 16-cell stage the sea urchin embryo is
composed of three different cell types, the
micromeres, mesomeres, and macromeres.
Each cell type gives rise to a particular embryonic structure (Horstadius, 1949). For
example, the micromeres are involved in
spicule formation. Methods are available
for dissociating and isolating each of the
three cell types as pure cell suspensions
(Spiegel and Rubenstein, 1972; Whiteley et
al., 1975). These can be reaggregated and
some of the resulting embryos will grow to
maturity. Figure 5 shows three Arbacia
formed from reaggregates. Lytechinus also
reaggregates and some of the plutei will
also grow to normal mature larvae that
metamorphose into feeding urchins. With
the use of different genetic strains, it will
now be possible to produce allophenic larvae and sea urchins.
REFERENCES
Barrett, D., and G. M. Angelo. 1969. Maternal characteristics of hatching enzymes in hybrid sea urchin
embryos. Exp. Cell Res. 57:159-166.
Birkeland.C, F. S. Chia.and R. R. Strathmann. 1971.
Development, substrate selection, delay of
SEA URCHIN DEVELOPMENT
metamorphosis and growth in the sea star, Mediaster
aequalis Stimpson. Biol. Bull. 141:99-108.
Brandriff, B., R. T. Hinegardner, and R. Steinhardt.
1975. Development and life cycle of the parthenogenetically activated sea urchin embryo. J. Exp.
Zool. 192:13-24.
Bury, H. 1895. The metamorphosis of echinoderms.
Quart. J. Microsc. Sci. 38:45-137.
Cameron, R. A., and R. T. Hinegardner. 1974. Initiation of metamorphosis in laboratory cultured sea
urchins. Biol. Bull. 146:335-342.
Costello, D. P., M. E. Davidson, A. Eggers, M. H. Fox,
and C. Henley. 1957. Methods for obtaining and
handling marine eggs and embryos. Marine Biological Laboratory, Woods Hole, Mass.
Czihak, G. 1975. The sea urchin embryo. Biochemistry and morphogenesis. Springer Verlag, Berlin.
Davidson, E. 1968. Gene activity in early development.
Academic Press, New York.
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