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2. The Early Cleavages.
In ophiuroids with small, comparatively non-yolky eggs the plan of cleavage followed is of the usual echinoid type—i.e., the first cleavage is equal, as also is the second, and both are vertical. The third cleavage is horizontal, and the upper and lower quartets are equal. The fourth cleavage is again vertical and more or less equal, and after that the cleavages occur fairly evenly over the whole embryo to produce a morula. The latter rapidly gives place to a regular blastula. These stages are represented in Figs. 2 to 6.
In the case of the moderately yolky eggs of ophiuroids, Grave (1916) has described the cleavages of the egg of Ophiura brevispina. His account shows that no difference is to be observed from the plan followed in the non-yolky group. He concludes, therefore, that the early stages of development in this species have been disturbed little if any by the increase in the yolk mass that has occurred.
Coming now to the yolky group having very large eggs, we have such forms as Ophiomyxa brevirima and Kirk's ophiuroid. The type of cleavage followed here has been described by the writer (1941) in the latter species. Cleavage in this case follows a somewhat different pattern to that seen in the two preceding groups, but the alteration nevertheless is not very profound. The first cleavage (Fig. 7) is unaltered.* The same applies to the second cleavage
[Footnote] * In about 30% of cases, however, an abnormal cleavage occurred in which the first two blastomeres were unequal, the larger undergoing two divisions to form a normal four-cell stage.
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Figs. 2–11.—Diagrams illustrating the effect of increasing yolk mass on the cleavage of the egg.
Figs. 2–6.—Cleavage in ophiuroids with small or moderate yolk mass (e.g., Ophiothrix, Ophiocomina, Ophiura).
Figs. 7–11.—Corresponding stages in cleavage of Kirk's ophiuroid, which has heavily yolked eggs.
(see Fig. 8). When, however, the third cleavage is reached, two quartets are formed which show a marked differentiation into micro-meres and macromeres (Fig. 9). This fact indicates that a prelocalisation must have occurred during the four-cell stage, or even earlier. As shown in my paper (1941), the macromeres are destined to form mes-endoderm, and are ventral (aboral) in position. The micromeres form the ectoderm and its derivatives, and in addition contribute to the mes-endoderm. This will be further discussed in the section of this paper dealing with gastrulation. In the cleavage immediately following the third, the division of the blastomeres becomes irregular and “out of step”. Finally, a morula with very turgid blastomeres is formed (see Fig. 11).
The changes to be observed in the plan of cleavage of this very yolky form are undoubtedly to be correlated with the great increase in the yolk material. It is interesting to note that, despite the comparatively enormous quantity of yolk present in Kirk's ophiuroid as compared with Ophiothrix or Ophiocomina, the plan of segmentation is really but slightly altered. One might, for instance, have expected to find some tendency to adopt partial cleavage, or to form a blastoderm, as has occurred in other groups where increase in the
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yolk mass has occurred. Indeed, such a supposition was put forward by Dr. Th. Mortensen in a private communication to the writer; but so far no evidence of such alteration in development has been found.
It would appear that the reason for this retention of the primitive type of division may be correlated with the fact already shown in this paper that the cytoplasm mass has increased in step with the increase in the yolk mass. Again, we may consider that the earliest stages of development are always the most rigid, and the last to be affected by factors producing ontogenetic change. This seems to be indisputable, for, as will be shown below, the later stages come to be tremendously altered in the heavily yolked forms.
One further effect of the increased yolk-mass in Kirk's ophiuroid remains to be noted. This concerns the alteration in the rate of cleavage. In my account of the development of this species, a table was given showing the progress of cleavage during the first twenty-three hours following the first cleavage. From this it was clear that a pronounced retardation had occurred, for whereas other ophiuroids with non-yolky or moderately yolky eggs reach the blastula stage at the end of the first 24 hours, Kirk's form is at that time only at the morula stage. Not till the end of the second day is the blastula completely formed, while the gastrula stage cannot be regarded as complete till the fifth day. On the other hand, other ophiuroids (Ophiothrix and Ophiura) have formed the gastrula at the thirty-sixth hour. Ophiocomina, as described by Narasimhamurti (1933) is slightly slower, forming the gastrula at from the fortieth to the forty-eighth hour.
fig. 12.—Time after fertilisation, in hours.
This retardation is illustrated in graphical form in Fig. 12. As the alteration in developmental rate is associated with increase in the yolk-mass, we can justifiably correlate the two conditions. That the presence of a large quantity of inert nutritive material should have such a delaying action on the ontogenetic processes is not surprising.