MUSEUM OF COMPARATIVE ZOOLOGY. 409 



vesicle divides and gives rise directly to the nuclei of the segmentation 

 spheres are, however, capable of another explanation, and such cases are 

 therefore not to be considered as exceptions to the ordinary course of 

 events. The trouble lies in the confounding of two entirely different 

 morphological structures, — the germiuative vesicle and the egg nucleus 

 (Eikern). The former possesses a firm membrane, fluid contents, and a 

 compact germinativ^ dot ; the latter is without membrane, homogeneous, 

 and without a nucleolus. As it is the latter which has been observed to 

 divide, Hertwig concludes that one may already affirm with certainty 

 that at the maturity of the egg the germinative vesicle as a morphological 

 structure perishes. To say how the " Eikern " originates is more difficult. 



A cardinal point is touched by the question, Does the q^^ exist at a 

 definite stage of its development as an enuclear yolk mass, — a cy tode % 

 The evidence furnished by those who maintain that such is the case, 

 Hertwig endeavors to weaken, by showing that, although a germinative 

 vesicle could hardly be overlooked, it would be quite easy to pass unno- 

 ticed so small and little diff'erentiated a structure as the germinative 

 dot, — the more, since anything like a satisfactory conclusion can only be 

 reached by having recourse to various reagents, and especially to meth- 

 ods of staining. His own observations have shown conclusively that in 

 some apparently very carefully studied cases a nuclear structure was 

 really present, and had been overlooked by his predecessors. Moreover, 

 in most cases the possibility of the persistence of the germinative dot had 

 not been sufficiently impressed upon the observer to make the observa- 

 tions certain on this point. Even in those cases (Auerbach and Stras- 

 burger) where a direct observation of the origin of the new nucleus is 

 claimed, it is not impossible that a very small germinative dot may have 

 remained unobserved and been in reality the initial stage of the supposed 

 new structure. The evidence, then, in favor of a new origin for the 

 " Eikern " is insufficient ; for, on the one hand, it is not established that 

 the egg passes through the cytode condition, and, on the other, the posi- 

 tive statements that the nucleus is a new creation are capable of another 

 explanation. Another error is coupled with this ; namely, that fecunda- 

 tion is the cause of the disappearance of the germinative vesicle and of 

 the formation of a new nucleus. " Der Schwund des Keimblaschens und 

 die Entstehung des Eikerns sind vielmehr Vorgange, die einzig und allein 

 mit der Reife der Eier zusammenhangen und die Befruchtungsfahigkeit 

 derselben herbeifiihren." 



Finding no entirely insurmountable obstacle in the literature, and sup- 

 ported by his own observations, he draws the general conclusion that " in 



