THE CELL DOCTRINE. 137 



more recent observers, Auerbach, Strassburger, 

 Oellacber, Kleinenberg, Biitschli, Balfour, and Van 

 Beneden, in accordance with fresh researches made 

 on account of the doubts which those of Oellacber 

 and Kleinenberg had created in his mind, all agreed 

 that the germinal vesicle disappeared entirely either 

 before or during fertilization. Hertwig and V-an 

 Beneden both furnish a carefully detailed account of 

 the steps in the destruction of the germinal vesicle, 

 the former in the unripe ova of Toxopneustes lividus, 

 and the latter in the ovum of the rabbit. While 

 agreeing in many particulars, they differ in their ac- 

 count of the final dissolution. According to Hert- 

 wig, the whole structure, after having become periph- 

 eral, disappears, leaving behind, of its contents, the 

 germinal spot, or nucleolus, which becomes the nucleus 

 of the ovum ready for the changes of fertilization. 

 Van Beneden, on the other hand, believes that all the 

 constituents of the germinal spot disappear in ioto. 



It seems also that the view of Hertwig, that a part 

 of the germinal vesicle remains to form the nucleus 

 of the ovum, was previously suggested by Derbes and 

 Von Baer. The former, in lb47,* described the 

 ovarian ova as consisting of three zones, the germi- 

 nal spot, the germinal vesicle, and the yolk, of which 

 the middle one only disappears. Von Baerf also 

 states that in the case of echinoderms, the germinal 



* Derbes, Observ. sur le Mocanisrae et le Phen. qui accomp. la 

 Formation de I'Embryon chez I'Oursin Comestible, Ann. des Sc. 

 Nat., Zoologie, 1847, vol. viii. 



■f C. B. V. Baer, Neue Untersuch., iiber die Entwickel. der 

 Thiere, Froriep's Neue Notizen, vol. xxxix, 1846. 



12* 



