A Remarkable Case of Animalization in a Batch of
Eggs of Paracentrotus lividus
by
SVEN HORSTADIUS 1 and TRYGGVE
GUSTAFSON2
Dedicated to
PROFESSOR ALFRED KUHN
on his 70th birthday, 22nd April 1955
eggs generally develop quite normally after artificial fertilization.
Sometimes it may be difficult to obtain good membrane elevation, but, after
repeated washing in sea-water, fertilization and further development may be
quite normal. In some batches of eggs abnormalities may, however, be observed
in the pluteus stage, but these abnormalities, as a rule, do not change the pattern
of the larvae profoundly. The defects are generally restricted to the arms and to
the skeletal rods which, for example, may fail to grow out to their normal length,
or may bend in an abnormal direction. Supernumerary rods may also appear,
and rods may grow in an abnormal direction inside the larval body without
causing a change in the exterior of the pluteus. During many years of experimental work with sea-urchin eggs, for the senior author dating thirty years back,
we have never encountered, nor have we seen in the literature, examples of such
an abnormal development as the one described in this paper.
In a batch of eggs of Paracentrotus lividus obtained in August 1954 at Roscoff
we found that about 1 per cent, of the larvae did not gastrulate at all (Text-fig.,
I—III), or only formed a very small archenteron (Text-fig., IV-VII). In all cases
a ciliary band was well developed, and as a rule a stomodaeum also, although the
latter was missing in a few cases (Text-fig., I). In more than 50 per cent, of these
abnormal larvae a very small archenteron had invaginated, forming a single or
bipartite vesicle (Text-fig., IV-VII).
In order to interpret this abnormal development we refer to the fact that
exactly these types of larvae have been described before, but never from whole
eggs. They resemble larvae obtained by one of us (Horstadius, 1935) from more
or less animal egg fragments of Paracentrotus. Isolated animal halves do not
gastrulate, but develop into one of the following types: blastulae uniformly
covered With Cilia (type A), blastulae with a ciliated field and the rest of the wall
consisting of a pavement epithelium (type B), blastulae with a ciliated band (type
SEA-URCHIN
Authors' addresses:
1
Zoologiska Institutionen, Uppsala, Sweden.
2
Wenner-Grens Institut, Norrtullsgatan 16, Stockholm Va, Sweden.
[J. Embryol. exp. Morph. Vol. 4, Part 2, pp. 217-19, June 1956]
218
s. HORSTADIUS AND T. GUSTAFSON
C), or blastulae with both ciliated band and stomodaeum (type D). The explanation given for the genesis of these types runs as follows. The absence of the
TEXT-FIG. Camera lucida drawings of abnormal larvae (I-VII) of Paracentrotus lividus. VIII, an
isolated animal half of the same type as larvae II and III, drawn at the same magnification.
vegetal half causes a predominance of the animal properties, leading to an increase of the apical tuft and lack of differentiation of a thin epithelium and a
stomodaeum. The halves of type A are to be regarded as the most animalized.
In the other types the vegetal properties have been more strongly represented
and favoured, e.g. the development of both a ciliated band and a stomodaeum
(type D), a differentiation equal to the prospective significance of the material.
Animal halves giving the richer differentiation have been called 'subequatorial', because in some batches the third furrow, in the eighth-cell stage, is
situated below the equator. In such eggs the animal halves will contain more
vegetal material and consequently have stronger vegetal properties than halves
from 'equatorial' eggs. This explains the more normal differentiation. It must
be emphasized, however, that such a 'vegetal' development can also be obtained
in eggs which are strictly 'equatorial' in their cleavage. This indicates that the
balance between the animal and the vegetal gradients is not the same in all eggs.
A further proof of this is afforded by a study of fragments called 8 + vegi + 0
(op. cit), i.e. fragments containing not only the cells from the animal half (8
mesomeres in the 16-cell stage), but also the adjacent ring of eight blastomeres
(vegi) from the vegetal half in the 64-cell stage. This material, the animal half +
vegi, corresponds to the entire ectoderm of the pluteus larva. In this experiment
we have added some vegetal material to the animal half and the result is as
SPONTANEOUS ANIMALIZATION IN SEA-URCHIN EGGS
219
would be expected, that ciliated band and stomodaeum also appear in fragments
derived from 'equatorial' batches of eggs. 'Subequatorial' fragments show a
more vegetal differentiation even if the third cleavage furrow is equatorial, as
a small archenteron and even skeletal spicules appear (op. cit, fig. 25).
It is now of particular interest that the abnormal larvae obtained from whole
eggs (Text-fig., I-VII) have their exact counterparts in animal fragments
described in the paper of 1935. Only the size is different. Our blastula with
ciliated band (I) is of the same type as the isolated animal half (fig. 12c, op. cit.).
II and III correspond to the animal halves in fig. 13 a-c, or the 'equatorial'
8 + vegi + O-fragments in fig. 24 f-h. A 'subequatorial' fragment like that in
fig. 25a is similar to our larvae IV-VII.
The differentiation of these strange types cannot be due to a dropping off of
the vegetal quarter of the egg in an early cleavage stage. The eggs developed
inside their fertilization membrane, and no small larvae of any kind were found.
Furthermore, the size indicates that the larva originates from a whole egg: compare the animal half (Text-fig., VIII) of the same type as II or III and drawn with
the camera lucida at the same magnification.
The explanation must be that in these eggs the vegetal properties have been
exceedingly weak compared with the animal ones. The gradient system in the
whole egg has been of about the same constitution as normally found in fragments representing only the presumptive ectoderm (8 + vegi + 0). The cell physiological background for this situation is unknown. It may be mentioned in this
connexion that about 1 per cent, animalization has been obtained in eggs of
Paracentrotus treated for 10 hours before fertilization with calcium-free seawater. Eggs from different localities reacted differently (Lindahl, Swedmark, &
Lundin, 1951). It is quite possible that in this case the animalization was the
result of the long treatment of the eggs with a calcium-free medium. It is interesting to find that the same effects occur spontaneously in eggs fertilized without
any delay and kept in normal sea-water.
SUMMARY
In 1954 at Roscoff some larvae from a batch of eggs of Paracentrotus lividus
were found to develop in the same way as eggs from which the entire endo- and
mesodermal region had been removed. The effect occurred quite spontaneously.
REFERENCES
HORSTADIUS, S. (1935). Uber die Determination im Verlaufe der Eiachse bei Seeigeln. Pubbl.
Staz. zool. Napoli, 45, 251-429.
LINDAHL, P. E., SWEDMARK, B., & LUNDIN, J. (1951). Some new observations on the animalization
of the unfertilized sea-urchin egg. Exp. Cell Res. 2, 39-46.
(Manuscript received 9: xi: 55)