CLEAVAGE IN THE EGG OF CEREBRATULUS. 11 



of the uornial egg {cf. Fig. 24). The blastulas are often open at 

 tlie vegital pole.^ 



From what has been described in the foregoing and this sec- 

 tion, we arrive at a conchision of considerable interest that w^iat- 

 ever tiie number of initial blastomeres (four in the normal egg, 

 six in the trefoil egg and eight in the Ca-free sea-water egg) the 

 mode of cleavage of each remains unaltered. Further detailed 

 study of the cytogeny of multiple cleavage is very desirable, for 

 instance, with a regard to the teloblast formation. Conklin ('07) 

 has made similar observation in the egg of Orepidula plana. He 

 compressed the egg and found that the third cleavage may divide 

 one or more of the macromeres equally, thus giving rise to five, 

 six or eight macromeres. If the pressure is removed from such 

 eggs, each macromere gives rise to micromeres in a manner approxi- 

 mately normal. From this he concludes that the number of 

 micromeres depends upon the number of macromeres and that the 

 omission or the addition of cleavage does not alter the character of 

 localization which determines the character of cleavage. As regards 

 the sign ill cance of this phenomenon I shall speak later. 



IX. Summary. 



The results obtained from the experiments upon the egg of 

 Cerebratulus lacteus are as follows : 



1. The mode of cleavage is not disturbed by the removal of a 

 portion of cytoplasm, unless the 02:>eration is performed close to 

 the beginning of the first cleavage. 



1 In the figures referred to, one notices that the third quartet was given ofi dexiotru- 

 pically as in the second. But this, I think, may be taken for an exceptional case. 



