STUDIES IN MAMMALIAN EMBRYOLOGY. 543 



Beneden, and being apparently also doubtful whether van 

 Beneden's definition of what is the anterior and posterior 

 region of the embryonic shield was accurate, he places a ? 

 behind the word '^ anterior ^^ with which he indicated this border 

 region, and which I am inclined to homologise with the 

 region of the protochordal plate in stages 52 and 73 of the 

 shrew. 



Moreover, comparing Kolliker's pi. i, fig. 1, on which an 

 embryonic shield is figured which very strongly resembles our 

 fig. 32, pi. c, of the shrew's stage 73/, we in the first place 

 notice the presence of the first trace of the gastrula ridge at 

 the lower border of the embryonic shield. It seems impro- 

 bable that from here the primitive streak should grow forwards 

 as Kolliker interprets it. Fig. 3 would no doubt seem to 

 militate in favour of such an interpretation, but then we must 

 not forget that it is not drawn on the same scale of enlarge- 

 ment. Were we to draw it on the same scale as fig. 1 the 

 embryonic region in front of the gastrula ridge would be of 

 about equal size to that of fig. 1, and we might look upon the 

 gastrula ridge as having arisen by a backward extension of 

 the original posterior proliferation hw. of fig. 1, together with 

 a general growth of the epiblastic shield much in the same 

 way as we have been led to interpret the phenomena in the 

 shrew. 



A comparison of Kolliker's figs. 4 and 5 with our figs. 

 33 — 53 of embryos 73, a, b, d, and of his figs. 7, 8^ with our 

 figs. 62 — 64 of embryos 45, and figs. 79 — 81 of embryos 42, 

 will also prove instructive. It must be borne in mind that 

 Kolliker's drawings are made from surface views, whereas 

 mine are reconstructions, and as such — though somewhat less 

 reliable as far as the outline goes — more exact where they in- 

 dicate histological differences. Kolliker himself says (1. c.^ 

 p. 23), "I have made no special study of the differentiation 

 of the cells of the germinal layers, nor of the karyokinetic 

 processes in them." We further notice that in Kolliker's 

 fig. 29 there is a central portion of flattened hypoblast, right 

 and left of which a histological modification is apparent. 



