555 



After fertilization by the sperm cells, which enter the substance 

 of the egg tail foremost, the yolk undergoes total segmentation. 

 The next step is the invagination of the ectoderm, a true Gastrula 

 state resulting. Fig. 217, A (after Kowalevsky), represents the 

 Gastrula; 7i, the primitive digestive cavity ; <>, the primitive open- 

 cavity (n) appears with an external opening. This cavity is 



formed by a union of two ridges which grow out from the upper 

 part of the germ. This is the central nervous system, and in the 



see, says Kowalevsky, a complete analogy in the mode of origin 



The next important stage is the formation of the tail. The pear- 

 shaped germ elongates and com racl a poster* >r\y until of theform in- 



