552 A. A. W. HUBRECHT. 



for the opossum, nor for that observed by Keibel in the rabbit, 

 and to which I will refer lower down, but rather to the less 

 markedly perforated hinder region of the embryonic shield of 

 Sorex figured in PI. XXXVIII, fig. 39. It should be especially 

 kept in mind that Heape's fig. 31 has been preceded by another 

 stage (his figs. 10 and 30), in which the hypoblast was already 

 completed and an independent closed sac inside the epiblastic 

 vesicle, so that their fusion and the perforation represented in 

 fig. 31 are no doubt already a secondary connection, as is the 

 fusion of the figs. 37 and 38 of Sorex. This early perforation 

 before the protochordal wedge has as yet very far advanced is 

 no doubt a peculiar feature which should be submitted to 

 repeated observation, but it is in no Avay an obstacle to the 

 theoretic views here given concerning the fusion of a palin- 

 genetic and cenogenetic hypoblast. Moreover another dif- 

 ference between Talpa and Sorex runs parallel to it, viz. the 

 greater distinctness and width of the neureuteric perforation 

 (see about this p. 532). 



Finally, an observation of Keibel in the rabbit deserves 

 more particular consideration with respect to this matter. 

 He represents (17) in his section, figs. 46 a, 466, and 47, a 

 stage which very nearly approaches what Selenka figures in the 

 opossum. Keibel draws attention to this correspondence, as 

 also to that between these stages and that of the mole according 

 to Heape. I have above given my reasons for which in this 

 latter respect I am inclined to difi'er from his interpretation ; 

 but I think we are fully justified in looking upon the spot bp 

 in Keibel's figures as the true cenogenetic blastopore of the 

 rabbit, homologous to that of the opossum. 



From Keibel's text I gather with satisfaction, as I have 

 already had occasion to notice above (p. 520, foot-note), that 

 he is not adverse to the view which I first put forward two 

 years ago, and have more fully advocated in this paper, viz. 

 that the formation of the mammalian hypoblast (i. e. the 

 phenomenon of gastrulation) is accomplished in two stages 

 separated by a short interval. I cannot, however, acquiesce 

 to his suggestion (1. c, p. 376) that in the first of these two 



