286 GENERAL EMBRYOLOGY 



when, once started on its course of differentiation, return or 

 ^differentiation in another direction is impossible. 



The formation of localized germinal areas of cytoplasm is to 

 be regarded as a process of development, and in the eggs of 

 different species this process may be carried forward at rela- 

 tively different times with respect to fertilization, cleavage, and 

 other early developmental phases. The most important steps 

 in cytoplasmic localization of the germ may be completed while 

 maturation and fertilization are going on, prior to the first 

 cleavage (Ascidians); or localization may be accomplished 

 during cleavage (Cerebratulus) , or not until the gastrula or post- 

 gastrula stages (Echinoderms). 



This idea is not essentially different from that of post-genera- 

 tion in certain respects, for regeneration and regulation are 

 after all essentially processes of development, deferred develop- 

 ment. The two differ however in that, according to the former 

 view localization is really present throughout the early stages 

 and disturbances are followed by an active process of regula- 

 tion; according to the latter, localization is not determined dur- 

 ing the earlier stages and when it does appear, the parts of the 

 egg remaining after the removal or injury of parts, behave as a 

 complete and normal unit, no regulation being necessary. 



There is evidence for both of these views, and both may be 

 true at the same time. The second appears to be the more 

 widely applicable. Regulation seems more likely to occur 

 during comparatively late phases of localization. Evidence of 

 regulation following the separation of blastomeres is afforded 

 by such eggs as those of the Echinoderms, where the isolated 

 blastomere continues to segment for a time as if it were part 

 of a normal cell group, but gradually its cell products assume 

 the characters of a typical whole group and finally give rise 

 to a normal embryo and larva. In other cases (Amphioxus) 

 separated blastomeres develop from the beginning' like whole 

 eggs, and no regulation is necessary. The results of deforma- 

 tion by pressure also indicate that localization is subject to a 

 regulatory process which may occur even in a comparatively 

 late stage in cleavage. 



