EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 47 
the two germ-layers of the mammals are also present in the 
Amphibia, and whether the mutual relations of these pro- 
liferating centres and the further fate of the tissues and 
organs they produce also reveal close similarity. 
We begin with the Gymnophiones, about the early phases 
of which A. Brauer (’97) has published an exceedingly lucid 
exposition based on the study of an extensive material. 
We have successively to look out for a proliferation of the 
entoderm corresponding to our protochordal plate, for an 
ectodermal proliferation representing the protochordal wedge, 
and for another ectodermal centre of growth which gives rise 
to ventral mesoblast. I will by means of copies of Brauer’s 
figures show that all three are met with in Hypogeophis and 
that in their further relations and in the genesis of the 
organs produced by them the homology with the mammals 
is indubitable. 
It should at the outset be remembered that the Hypogeo- 
phis egg is so saturated with yolk-material, that there is no 
holoblastic cleavage, but that the results of the cleavage pro- 
cess—as was noted jn Chapter IJ—are found at one pole of 
the egg, and that a\process of delamination transforms the 
fragmented ovum without delay into a gastrula with an 
entodermic lower layer (see Brauer, ’97, Figs. A and B, 
pp. 405, 404). 
More or less simultaneously a differentiation of ectoderm 
and endoderm becomes visible, which shows very great simi- 
larity with what was figured above (Fig. 19) for Tarsius. 
The point at which the ectoderm has commenced to proliferate, 
and at which its first change was a bend downwards (Fig. 83) 
is directly comparable to the primitive (Hensen’s) knob on the 
mammalian ectodermal shield, and is no other than our pro- 
tochordal wedge. The point at which the endodermal pro- 
liferation becomes evident is situated just in front of it and 
the two proliferations, as is so particularly clear in Brauer’s 
fig. 43, here reproduced as Fig. 84, fuse in absolutely the 
same way as we see in the ''arsius, Fig. 48; with full confi- 
dence I indicate the corresponding regions in the Amphibian 
