10 



ART. ]0. — N. YATSU : 



22^ ^ 25af 



Fig. IV. {Cerehratuhis marginatum 14-23; C.ladeuf^. 24). 



19-23, early cleavage stages of trefoil ei;gs'. 19, trefoil egg at the end 

 of the first cleavage ; 20, 6-cell stage seen from the vegital pole ; 21, 6- 

 cell stage in profile ; 22, 12-cell stacre ; 23a-o;, seven sta<>;es of a trefoil ei^g 

 (12-42-cell stage). The heavy lines separate the blastoineres of two sex- 

 tants ; 24, 32-cen stage of a normal egg. 



The trefoil 2g^^ cleaves like that of ecliinoderms described by 

 Morgan ('9.">, p. 7o) and Bovert ('07, p. '2ö). The second cleavage 

 is vertical resulting in a 6-cell stage (Figs. 20 and 21). The third 

 cleavage is horizontal as in the normal egg (Figs. 22 and 23a). 

 The subsequent cleavage (Fig. 23 b-g) shows clearly that each 

 blastomere of the 6-eell stage behave like that of the 4 cell-stage 



