36 A. A. W. HUBRECHT. 
evanescent blastopore of the didermic gastrula stage of 
mammals. However, only in some few mammals has this 
blastopore been shown to appear as an actual though very 
temporary and evanescent perforation of the embryonic shield 
(Figs.53—57). The proliferation has been known by the name 
of its first observer as ‘‘ Hensen’s knob;” it has also been 
called the primitive knob (Bonnet, ’89, pp. 38 and 40) ; for 
myself I wish to adhere to the name I have proposed for it 
many years ago (790, p. 501) and call it “ protochordal wedge,” 
as I have called the entodermal proliferation “ protochordal 
plate.” The point of importance in my wishing to stick to 
these names is that the next step in mammalian development 
is the firm fusion between these two independent prolifera- 
tions that have arisen in quick succession in the two 
independent germ-layers, and that will henceforth be no 
more disconnected (Figs. 47, 48, 52, 57, 97—99). ‘I'he noto- 
chord is built up of material situated in the axial line of these 
proliferations, hence the names. 
Already Hensen has correctly observed (’76) that below 
the rounded knob which he found projecting downwards 
from the ectoderm, the degree of firmness with which ento- 
derm and ectoderm cling together is at its maximum, and 
should be looked upon as an effective fusion of the two 
layers. ‘This is fully confirmed both by transverse and longi- 
tudinal sections. I found the same in the shrew (Fig. 57) and 
more lately, in a yet higher degree, in Tarsius (Figs. 48 
and 52). 
In 'T'arsius where we have already seen on p. 32 how very 
massive the protochordal plate was, the protochordal wedge 
pushes downwards just behind it, over that part of the ento- 
derm which again consists of flattened cells. The fusion 
between the proliferated endoderm and ectoderm cells, not 
yet effected in the section of Fig. 49, comes about immediately 
afterwards (Fig. 48). There is not the slightest evidence in 
Tarsius that the knob-like ectodermal proliferation which we 
have called the protochordal wedge undergoes any extension 
forwards which could be identified with what German authors 
