40 A. A. W. HUBRECHT. 
8, Mutual Relations between the Centres of 
Proliferation. 
We must now consider the relation in which on the embry- 
onic shield the centre of proliferation of the ventral meso- 
blast stands to that, which we have designated as the proto- 
chordal wedge. In general it may be said that in the earlier 
stages the former lies immediately behind the latter. We 
may add to this that if Tarsius were possessed of a blastopore 
in the didermic gastrula in the same way as Hrinaceus is (and 
as are some other mammals) the situation of this blastopore 
would be such as to separate these two centres of prolifera- 
tion. This becomes evident when we consider the exceptional 
case already noted above (p. 14) where the embryonic shield 
of a particular specimen of Tarsius was provided with a deep 
pit-like impression (Fig. 52) which cannot but be looked upon 
as an attempt at blastoporic perforation of atavistic sigmifi- 
cance, the very numerous stages of Tarsius of identical age 
which I have in my possession not revealing a trace of it. 
Other cases in which the contiguity, but at the same time 
the mutual independence of the two centres of proliferation 
is evident were figured by me in a former publication (’02, 
fis. 58b, 46d, 47, 48,52, b and c).1 Out of them all (see also 
Figs. 47, 48, 49, and 50) I have constructed the semische- 
matic diagrams, Figs, 44—46, 
IT need hardly explain that the presence, close to each other, 
of three centres of proliferation (one entodermal, two ectoder- 
mal) in the two germ-layers of the mammals, such as we have 
just described, combined with the fact that in each centre new 
cells are very actively developed which spread out in the only 
direction available to them, i.e. between these two germ-layers, 
1 J will here notice that the mutual independence here insisted upon should 
be taken cum grano salis. The anterior and the posterior lip of the 
blastopore, being naturally connected by the lateral lips, it is not a material 
anatomical independence that is here meant, but an independent activity. On 
p. 44 this will be more fully entered into. 
