548 A. A. W. HUBRECHT. 



facts in the opossum on a Jevel with Bonnet^s and my own 

 results in placental mammals. 



Thus on Selenka^s fig. 2, pi. xxi, the front end of the 

 notochord in its very first stages is figured. A thin layer 

 with nuclei beneath it should be erased in this figure, as we 

 are told on p. 151 of the text. When this erasion has been 

 brought about, the figure very strongly suggests the identity 

 of the massive plate of 7 — 10 thick entodermal cells, with the 

 protochordal plate of our figs. 66 and 67. It seems to me 

 hardly possible here to adopt van Beueden's or Carius' views, 

 and look upon this portion as the widened anterior part — 

 intercalated in the hypoblast — of the " Kopffortsatz." The 

 front end of the latter is perhaps situated in Selenka's fig. 4 

 (or 3). If we now consult his surface view (fig. 1, pi. xxi), 

 we see that just in the level of section fig. 4 the notochord is 

 very considerably constricted. Later researches Avill have to 

 make out whether the two regions, anterior and posterior to 

 this spot, may be identified respectively as protochordal 

 plate and protochordal wedge. Of the very earliest phases 

 of the gastrula ridge Selenka does not give any figures, 

 nor do his figs. 8 — 11, pi. xviii, furnish material for a 

 profitable comparison with those very earliest stages in 

 Sorex. 



A very emphatic opponent to some of Bonnet^s views, par- 

 ticularly those which refer to the formation of an annular zone 

 of hypoblast, from which meso blast (on purpose I do not follow 

 Bonnet in the use of the term mesenchyme) is originated, is 

 found in Fleischtnann, who has more especially occupied himself 

 with the development of Carnivora. As we have been able to 

 show that in the shrew Bonnet's results are fully substantiated, 

 and to bring forward additional evidence in karyolitic figures 

 which are not figured by Bonnet, we necessarily find ourselves 

 in conflict with Fleisclimann. This author (' Unters. iiber 

 einh. Baubthiere,' 1889, p. 17) does acknowledge the possi- 

 bility that in this respect fundamental differences between the 

 Carnivora and the mammals which Bonnet and myself have 

 studied exist. Still his a priori argumentation against the 



