640 H. GRAHAM CANNON 
and also in Daphnia Agar (2) has shown that while the 
embryo is in the brood-pouch it does not receive nourishment 
from its parent. And yet §. vetulus shows a type 
of development which differs considerably from that of 
Daphnia in that there is a very early segregation of the 
genital rudiment but shows such obvious similarities to the 
development of Moina and Polyphemus. 
The fact that has been pointed out by Fuchs (4), that among 
sroups so far apart as the Copepoda and the Cladocera, m 
forms where there has been loss of yolk owing to the develop- 
ment of other modes of nutrition of the embryo, there is such 
an extraordinary similarity in the cell lmeages, suggests firstly 
that the arrangement of the ‘ Anlagen’ in the eggs of these 
forms is a very archaic character, and secondly that in cell 
lineage there is a representation of the arrangement of the 
‘Anlagen’ in the yolky eggs that do not show a teloblastic 
mode of development. ‘This view is upheld by the similarity 
between the cell lineages of the Cladoceran eggs that contain 
little yolk and of the egg of the Cirripede Lepas, where 
although there is abundant yolk, yet there is determinate 
cleavage (Bigelow, 8). In the development of 8. vetulus 
there is further support of this view in that in this apparently 
indeterminate method of development the earlest arrangement 
of the germ-layer ‘ Anlagen’ shows such a close resemblance 
to the arrangement of the teloblasts in the non-yolky eggs of 
the Cladocera. 
7. SUMMARY. 
1. Each egg is laid as a yolky mass of a foam and later forms 
a primary egg-membrane. 
2. Cleavage is completely superficial and apparently indeter- 
minate. 
3. The first differentiation of the blastoderm is the appear- 
ance of a group of vacuolated yolky cells on the ventral side 
of the embryo which are called the ventral mass. 
4. This subsequently differentiates into a few large cells 
with very large nuclei which form the genital rudiment, sur- 
rounded laterally and posteriorly by ectomesodermal cells, 
