NUCLEI OF ANIMAL AND VEGETABLE CELLS. 149 



dom. The last-mentioned writer supposed that the germinal 

 vesicle is in no case abolished, but merely rendered for a time 

 invisible by reason of certain changes in the surrounding 

 yolk, and that it reappears before cleavage. 



According to the third view, of which Hertwig, among the 

 authors whose views we have been examining, is the repre- 

 sentative, the germinal spot is saved in the degeneration 

 of the germinal vesicle and appears as the nucleus of the 

 ovum ready for the changes which fertilisation will bring on. 

 This view of the continuation of the germinal spot is not 

 new. Derbes,! in 1847, described the ovarial ovum as con- 

 sisting of three zones, the germinal spot, the germinal vesicle, 

 and the yolk ; of which the middle one afterwards disappears. 

 Von Baer,^ again, in the case of echinoderms, states that the 

 germinal spot remains as a nucleus to the ovum when the 

 germinal vesicle is no longer to be seen. Similar state- 

 ments are also made by Leydig^ concerning Piscicola, and 

 by Bischoff ^ in the case of mammals. 



With regard to these views it seems impossible, as Hert- 

 wig points out, to doubt the correctness of those who assert 

 a disappearance of the germinal vesicle. To suppose that 

 such a conspicuous object as the germinal vesicle was over- 

 looked by them, becomes extremely difficult when the num- 

 ber and trustworthiness of the observers is considered. Hence 

 it must be concluded that the error lies with those who have 

 believed in its continuance ; for it is scarcely possible that 

 both sets of statements can be correct in the particular cases 

 to which they refer ; or, in other words, that such a marked 

 difference should exist between, let us say. Hydra on the one 

 hand (where the germinal vesicle disappears) and Medusae 

 (where the vesicle divides according to Gegenbauer) on the 

 other. The possibility of such an error is by no means remote; 

 for the descriptions of the vesicle as a homogeneous non- 

 nucleolated vacuole, which are given by the majority of the 

 observers, are such as to suggest strongly a confusion with 

 the nucleus of the primary cleavage mass, which is acknow- 

 ledged by all to undergo division. 



The view of a continuance of the germinal spot alone 

 admits of no such objection. Those who, like Auerbach and 



* Derbes : " Observ. sur le Mecanisme et les Pheii. qui accompagnent 

 la Formation de I'Etnbryoa cliez I'Oarsia comestible," 'Ann. des sc. nat. 

 Zoologie,' 1847, tome viii. 



' C. E. V. Baer : " Neue Untersuch. ii. die Entwicklung der Tliiere," 

 ' Frorieps' Neue Notizen,' Bd. 39. 



^ Leydig : " Zur Anatomic von Piscicola Geometrica," ' Zeitsch. f. wiss. 

 Zool., Bd. i. 



* Bischoff : "Entwicklungsgesch. des Kanincbeneies," 1842. 



