38 A. A. W. HUBRECHT. 
as soon as we come to know their development in these same 
early stages—will, in all probability, fully confirm what 
Tarsius teaches us, considering that in so many other impor- 
tant points ‘arsius is seen to resemble the other Primates 
most closely, and that in this very detail: the presence of an 
extra embryonic ccelom at a stage ever so much earlier than 
in any other mammal, there exists perfect uniformity. 
In Tarsius there is no doubt that before the appearance of 
the protochordal wedge (Hensen’s knob) in the posterior 
third of the ectodermal shield, another ectodermal prolifera- 
tion has already preceded Figs. 47—50, vm.), the products of 
which have important parts to play in the constitution both 
of embryo and blastocyst, different, however, from those of 
the protochordal wedge. 
This earlier ectodermal proliferation is primarily directed 
backwards (Fig. 49), whereas the protochordal wedge has a 
faint inclination forwards (Figs. 46 and 48). Like this it is 
median and unpaired. 
We will call this posterior proliferation the origin of the 
ventral mesoblast (Hubrecht, ’02, pp. 19 and 31), and we may 
emphasise that, whereas the wedge appeared in the hinder 
third of the ectodermal shield, this ventral mesoblast originates 
still further back (separated from the wedge by the potential 
blastopore) at the posterior extremity of the embryonic shield, 
there where the trophoblast is often quite sharply differenti- 
ated (cf. Figs. 48 and 49) from the embryonic ectoderm. We 
encounter this proliferation as soon as the entoderm after its 
delamination from the embryonic knob is busy forming a 
vesicle under the embryonic ectoderm (Figs. 44 and 62). This 
endodermal vesicle, as was seen. in the preceding chapter 
(p. 8), never fills the whole of the blastocyst. Now the 
proliferation at the hind end of the embryonic ectoderm, which 
we consider as the primordium of the ventral mesoblast, 
hollows out at the very earliest moment, thus forming a 
second vesicle enclosed inside the trophoblast. 
The cavity of this vesicle should be classed as extra-embryonic 
celom; its walls, where applied against the trophoblast 
