No. 2.] ANURIDA MARITIMA. 251 
been definitely determined, owing to their extremely small 
size, but the indications are that there are eight in each 
nucleus. Fig. 31 is a section through an 8-celled egg at 
the line z—-¢ in Fig. 21. It shows the distinct holoblastic 
cleavage and four of the cells. There is practically no cen- 
tral cavity, the cells being crowded in together. The nuclei 
and protoplasmic masses are already preparing for the 16- 
celled stage; as this division is in vertical planes, the nuclei 
divide in a horizontal plane and can be seen in this section. 
In Fig. 32, the 32-celled stage represented in Fig. 23 has 
been cut horizontally at about the level of the line y-y. 
This shows distinctly that the typical blastula does not result 
from the cleavage; some of the cells have been crowded into 
the interior so that a solid morula results. This condition is 
still more evident in sections of the typical morula stage 
shown in Fig. 24. Up to this point the cleavage has been 
undoubtedly holoblastic and practically equal, for the slight 
inequality shown in the 4-celled stage becomes more and 
more obliterated by subsequent divisions. After the morula 
has been formed a decided change takes place in the internal 
structure corresponding to the external features already de- 
scribed. In Fig. 33 there is shown a gradual obliteration of 
the hitherto distinct blastomeric outlines. The nuclei and 
protoplasm of the outer blastomeres have migrated to the sur- 
face, leaving the yolk masses on the inside. In the inner part 
the nuclei and surrounding protoplasm have entirely left the 
yolk masses and are evidently moving towards the surface. 
There has been a cessation of the total cleavage, and now the 
blastoderm is being formed by the migration of the cells from 
the blastomeres to the exterior. Consequent on this change 
the yolk is left behind as an inert mass; and, though at first 
retaining a separation into masses corresponding to the earlier 
blastomeres, it gradually assumes a more compact arrange- 
ment. The cells, if this name can be applied to the migrating 
masses of protoplasm containing a nucleus, divide as they pass 
to the exterior, and some remain behind, also undergoing the 
process of division. Fig. 34 shows the final result of this 
migration. Here a blastoderm has been formed by migration 
