248 



ZOOLOGV 



Fig. 236. — Illustrating the transformations from the egg to the youug 

 larva of an echinoderni (the sea-urchin, Arbacla). From W. K. Brooks. 

 a, first cleavage plane formed, 2-ccll stage ; h, second cleavage plane 

 formed, 4-cell stage ; c, third cleavage plane formed, 8-cell stage ; d, 

 \aew from one pole of the 16-coll stage ; c, same stage from the side ; /, a 

 later stage witli numerous cells ; g, same stage seen in section, the " blas- 

 tuia " ; /(, the transition to the gastrula stage ; i, side view of young larva. 



