522 A. A. W. HUBRECHT. 



process of resorption ; they leave no trace, and they can 

 certainly not be looked upon as palingenetic structures having 

 phylogenetic significance ! 



In the same way I am inclined to look upon a definite 

 rounded patch of hypoblast below the embryonic shield and 

 gastrula ridge as of cenogenetic significance, and I will now 

 point out certain morphological, histological, and develop- 

 mental peculiarities which substantiate this view, at least for 

 certain mammals, viz. the shrew here described and the sheep 

 (Bonnet). The patch to which I allude forms part of the 

 space which is enclosed inside the annular ring of modified 

 hypoblast, with the anterior part of which the prbtochordal 

 plate (which arises independently of it) afterwards fuses (see 

 p. 515). In figs. 33 — 35 the extension of the oblong patch 

 here alluded to is distinctly marked.^ In the sections the 

 hypoblast-cells of this region are seen to be uncommonly flat, 

 the nuclei very wide apart. 



It is with these cells that the lower layer of the gastrula ridge 

 fuses ; it is here that the so-called connection between epiblast, 

 mesoblast, and hypoblast comes about (cf. fig. 38), a connection 

 which in Mammalia is undoubtedly secondary. If my view is 

 correct the connection is one between primary (or palin- 

 genetic) and secondary (or cenogenetic) hypoblast, and we 

 can well understand on this hypothesis that the fusion becomes 

 so intimate that soon it is impossible to notice any boundary 

 line. Actual resorption of the flattened patch of secondary 

 hypoblast by the much more massive, bulky, and active cells 

 of the primary hypoblast (gastrula ridge) may, for aught I 

 know, take place. At all events, I have not noticed the 

 slightest fact in support of what Ileape has brought forward 

 for the mole, and Lieberkiihn and Hensen for other mammals, 

 that at the point of fusion between gastrula ridge and under- 

 lying hypoblast (it is the anterior, not the posterior region 



' Bonnet figures early stages of the sheep's blastocyst (' Arcliiv. f. Anat. u. 

 Phys.,' 1884, Anat. Abth., pi. ix, figs. 8, U, 15, 3G, and 38). Tlie light 

 space in the centre of his annular " Mesoblast-hol" corresponds to the 

 region which is figured for the shrew, and which is here alluded to. 



