IV. i INITIAL STRUCTURE OF THE GERM 



217 



the relation of the yolk-lobe to the raesoderm. When the former 

 is cut off the latter is not formed at all. There are four generations 

 of micromeres and four equal macromeres, and the larvae, though 

 ciliated and able to swim, never reach the veliger stage (Fig. 127). 



FIG. 124. Iltjnnassa : cleavage of CD. a, Formation of yolk-lobe 

 (yl) ; b, 'trefoil' ; c, % embryo from above ; d, early | embryo, from above: 

 secondary yolk-lobe (ylj ; e, resting embryo from above ; /, T 'J> embryo 

 from above ; y, passage to - t 8 g- stage, f from above ; h, \% embryo, from 

 side ; /, division of mesoblast pole-cell (d 4 ), from below. (After Crampton.) 



The potentialities of the blastomeres are thus strictly limited 

 both in segmentation and differentiation. This result is amply 

 confirmed by Wilson's researches on Dental'ntm and Patella, 



