388 BULLETIN OF THE 



These two views more or less closely reproduce the opposing ideas held 

 by the earlier embryologists. An extended review of those earlier opin- 

 ions is rendered unnecessary by the labors of recent investigators, espe- 

 cially Oellacher, Flemming, Fol, 0. Hertwig, Biitschli, Ed. van Beneden, 

 and Whitman. The diiferent views group themselves naturally under 

 one or the other of three heads, according as it was claimed that the 

 germinative vesicle disappears, or persists and undergoes division, or, 

 finally, that only the germinative dot remains while the vesicle suffers 

 dissolution. 



Although it has often occurred that the same author has arrived at 

 opposite conclusions in the study of different animals, it does not follow 

 that differences of opinion can in general be referred to the actual exist- 

 ence of differences from one species to another ; on the contrary, it has 

 been possible for the study of the same species to lead to the most diver- 

 gent conclusions.* 



But if the germinative vesicle does not persist and undergo division to 

 form the nuclei of the first pair of segmentation spheres, what becomes of 

 its substance 1 This question has had various answers. Purkinje ('30, 

 p. 15), the discoverer of the vesicle, entertained the opinion that, inas- 

 much as it was not to be found in (hen's) eggs taken from the oviduct, 

 it became ruptured by the contractions of that organ, and that its con- 

 tents, a " lympha generatrix," became mingled with the germ. It was 

 owing to this supposed germinative influence of the contents that the 

 vesicle was thus named. Von Baer ('27), who was the first to show that 

 a migration of the vesicle takes place in the hen's egg from the centre to 



* In the case of the rabbit, for example, BischofF {'42, pp. 38, 39, 75-77, 141) 

 concludes that the germinative vesicle, which exists as a protective envelope to the 

 germinative dot up to the time of impregnation, disappears, and that the dot under 

 the influence of the male element undergoes division, the two parts becoming tlie 

 centres about which the yolk is grouped to form the first pair of segmentation spheres. 

 Ed. van Beneden ('70, pp. 178, 179), on the contrary, holds that the disappearance 

 of the germinative vesicle is apparent rather than real, just as it is an untenable posi- 

 tion to maintain that the nuclei of segmentation spheres, because they become pale, 

 homogeneous, and transparent, disappear and are replaced by new nuclei. Also at 

 p. 244 (loc. cit. ) he says : " En resume je considere non comme demontre, mais comma 

 tres-probable que la vesicule germinative se divise au lieu disparaitre, et que ses por- 

 tions deviennent les noyaux des deux premiers globes vitellins." 



Bischoff ('45, pp. 22, 42, '52, pp. 20, 21) subsequently modified his views in so 

 far as to consider the persistence even of the germinative dot as problematic, and Van 

 Beneden ('76", pp. 39, 40, and '76^ p. 154) now goes so far as to assert that his "re- 

 searches on the ovum of the rabbit have proved .... that no morphological part of the 

 germinal vesicle is found in the yolk at the moment of fecundation." 



