544 A. A. W. HUBRECHT. 



which offers points of comparison with the modification by 

 which the annular zone {hy. ar., figs. 53, 54, 68) of the shrew 

 is characterised. Moreover, fig. 47 of Kolliker raises the 

 doubt whether also in the rabbit certain points of the hypo- 

 blast might not finally be detected, where its participation in 

 the formation of mesoblast could not be as decidedly demon- 

 strated as it has been done by Bonnet for the sheep and by 

 myself for the shrew. 



It does seem that the rabbit is in this respect a very recalci- 

 trant subject, if we notice how also Rabl {" Theorie des Meso- 

 derms," ' Morphol.-Jahrbuch,^ 1889, p. 113, pi. ix) confirms 

 the views as advocated by Kolliker. From Rabl's essay I will 

 notice certain details which should be compared with what I 

 have above advanced for the shrew. Rabl states that in an 

 early stage of the rabbit's blastocyst the two layers are at 

 first in no way directly connected, which corresponds with 

 what was observed in the shrew. 



Contrary to what we have noticed in Sorex, he states (1. c, 

 p. 144) that in the rabbit this mutual independence of the two 

 layers is maintained even after the gastrula ridge and proto- 

 chordal wedge of the rabbit have made their appearance, and 

 the formation of the mesoblast is already considerably ad- 

 vanced. Somewhat later, however (1. c, p. 149, pi. ix, 

 fig. 9), the fusion of the three layers in the region of Hen- 

 sen's knob comes about, although as yet to no considerable 

 extent. 



As to Rabl's figures (1. c, pi. ix, figs. 5 — 9), I must again 

 invite comparison with what was figured for the shrew on 

 PI. XL, figs. 66 — 78. These figures do not exclude the possi- 

 bility of seeing in the anterior part of his " Chordaplatte " a 

 portion homologous to our hypoblastic protochordal plate, 

 although, of course, Rabl's text goes the other way, and he 

 wishes to interpret them as confirmative evidence for van 

 Beneden's gastrulation hypothesis. 



