STUDIES IN MAMMALIAN EMBRYOLOGY. 523 



of the gastrula ridge that is here understood !) the latter takes 

 any part in the production of new cell material for the gastrula 

 ridge or the raesoblast. Karyokinesis of the nuclei forming 

 part of this thin stratum of hypoblast in a plane coinciding 

 with it was never noticed by me, and I am here in entire 

 accordance with Bonnet.^ However, if we carefully consider 

 the wording of Heape's conclusions {' Quart. Journ. Micr. 

 Sci./ 1883, p. 47), we find that he writes, ^^Immediately 

 below the primitive groove there is no layer of hypoblast to be 

 distinguished, and here mesoblast is produced from hypoblast- 

 cells (sic). Laterally all three layers are distinct, but in the 

 middle line they may be said to combine with one another, 

 and in this region, therefore, the middle layer is formed from 

 both epiblast and hypoblast." 



This proves that he deduces the participation of the under- 

 lying hypoblast in the front region of the primitive streak, 

 towards the formation of raesoblast, not from any karyolytic 

 indication, but from the mere fact that the distinction of the 

 three layers being in tliis region no longer possible, a partici- 

 pation of both the primary ones in the formation of the third 

 one cannot be strictly denied. 



Since then the progress made in the study of karyokinesis 

 has rendered us more careful, but at the same time more 

 decided. So that I hold that I may safely deny any active 

 proliferation, in the sense here alluded to, of the hypoblastic 

 layer that first underlies and afterwards fuses with the front 

 end of the gastrula ridge (primitive streak) ; and may, on the 

 contrary, assume that a process of resorption of this patch of 

 tissue is more in accordance with what we observe in sections. 

 This resorption also applies to the short stretch of hypoblast 

 underlying the protochordal wedge (see figs. 49, 55, 68, 74, 



1 Bonnet writes (1. c, 1889, p. 41), "Dafiir, dass dem Knoten oder der 

 Gastrulaleiste vom Entoblast her durch Theilung oder Ausschaltung der Ento- 

 blast-Zelleu Zuwachs geliefert werde, finde icli, audi uacli erneuter Revision 

 meiuer Sclinittserien, ebensoweiiig Aulialtspunkte wie v. Kolliker beim 



Kaninchen. Die miibsame Controle ergab stets dass die Tliei- 



lungsebene senkrecht auf der Entoblasifliiclie stand." 



