DEVELOPMENT OF EGG FRAGMENTS IN CEREBRATULUS. 135 



age of egg fragments was perfectly normal in Series A, B and C. 

 It is especially noteworthy that the nucleated pieces of the fertil- 

 ized eggs cleave exactly like a normal egg. Whatever amount 

 of cytoplasm be cut off, the nucleated piece always divides at first 

 into two and then into four equal blastomeres. Since these frag- 

 ments so often gave rise to defective embryos, it is probable, not 

 only that the two and four blastomeres differ from each other, 

 but also that they do not correspond with those in the normal 

 case. The same is true in the eight-cell stage, and so on. From 

 this it seems further probable that cleavage of an egg takes its 

 normal course irrespective of the localization of embryonic 

 regions within. The cleavage pattern is stamped on the cyto- 

 plasm ; but the end-result is governed by a different set of factors. 

 The cleavage factors seem, therefore, to differ in this case from 

 the morphogenic ones. " Though there is often a close and con- 

 stant connection in the normal development between the process 

 of cleavage and that of localization and differentiation, this con- 

 nection is not a necessary relation" (Wilson, I9O3 1 ). 



It is rather striking that, if the operation is done before the 

 completion of the first cleavage, most of the egg fragments give 

 rise to defective larvae, while all of the isolated blastomeres at 

 the two-cell stage develop into perfect pilidia (though the result 

 is not conclusive, since my cases^were too few). It is easy to 

 conceive, however, that natural cleavage comes into operation in a 

 quite different way from the artificial section, and it is probable 

 that by the natural cleavage all the organ bases are equally dis- 

 tributed into two blastomeres which would be very unlikely after 



an artificial section. 



SUMMARY. 



1. Before the germinal vesicle fades, there is no evidence of 

 definite specification in the egg regions. 



2. Dissolution of the germinal vesicle initiates the establish- 

 ment of the germinal localization. 



3. In the period between the entrance of the spermatozoon 

 and the fusion of the germ nuclei, the localization becomes more 

 definite. 



4. The basis of the apical organ is not at the animal pole, but 



1 Wilson, /. c. , p. 439. 



