258 University of California Publications. [Zoology 



right angles to the first, dividing each blastomere somewhat 

 unequally. A small cavity between the blastomeres at this stage 

 (Fig. 6, F) completely vanished a few minutes later (Fig. 6, G) 

 The third cleavage first affects the two larger blastomeres; and 

 the fourth may appear in the blastomeres thus formed before tht 

 two smaller cells of the four cell stage have divided. 



Amoeboid processes are evident at different -stages, but the 

 egg as a whole retains a definite form to the close of segmenta- 

 tion. It is then approximately spherical, and is closely envel- 

 oped by a delicate chitinous membrane. 



This stage is reached eight or ten hours after the egg is dis 

 charged from the parental manubrium. The embryo is then 



COCO 



G 

 Fig. 6. Early cleavages of Corymorpha egg. Time, oue hour. X 58. 



composed of a solid mass of yolk cells surrounded by a layer of 

 cells Avithout yolk granules. Among the former, one or more 

 cavities make their appearance. The peripheral yolk cells 

 gradually assume an epithelial character. Within twenty-four 

 hours, there is a single central cavity with epithelial walls which 

 at first is almost filled by yolk cells in process of disintegration 

 A columnar epithelial ectoderm is already extablished. 



lY. — The Larva. 



With the elongation of the body which now takes place, the 

 embryonic period may be said to end and the larval period to 

 begin. The latter presents certain peculiar features. The larva 

 never passes through a free, ciliated, planula stage, but behaves 

 from the beginning like a planula which has attached itself, lost 

 its cilia and is on its way toward the formation of a polyp. The 

 end which grows awav from the substratum becomes the oral 



