DEVELOPMENT! OF SIMOCEPHALUS 637 
shallow, while at other times, as stated above, it has been 
seen to stretch one-third of the way across the egg. This fact, 
when it is also remembered that the surrounding cells are 
actively proliferating and producing cells which push their 
way inwards between the edge of the genital rudiment and the 
blastoderm, suggests that the invagination may be brought 
about by the ectomesoderm cells pushing the genital rudiment 
Text-Eie. 1. 
Mesendodermal Cells Endoderm 
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Genital 
Rudiment 
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(4) (0) 
(2) Diagram of the ventral view of embryo of Simocephalus 
vetulus, showing the ventral mass before the formation of the 
mesendoderm., 
(b) Ventral view of embryo of Polyphemus pediculus in 
thirty-two-cell stage (from Kiihn), 
in front of them as they themselves pass into the embryo. 
A second difference lies in the fact that in Daphnia the 
primordial germ-cells when they have passed into the interior 
lose their yolk. Vollmer states (14): ‘auch in den Blastoderm- 
zellen schreitet die Dotterresorption fort, wenn auch nicht in 
demselben Grade wie in der Gonadenanlage’. In $8. vetulus 
the cells of the genital rudiment always consist of large yolky 
cells which retain ther yolk all through the development. 
Their protoplasm also stains very faintly, not as in Daphnia, 
where Vollmer states that these cells show an increased 
affinity for stains. However, from the position of origin of the 
NO. 260 uu 
