34 A. A. W. HUBRECHT. 
There is no doubt that to a great extent the mesenchyme 
here described contributes towards the formation of blood- 
vessels and blood. The protochordal plate furnishes the 
endothelium of the heart, as I have elsewhere demonstrated 
for Tarsius (702, Pl. IX, fig. 73, a and 6b), the annular zone 
produces the material for the area vasculosa on the umbilical 
vesicle. To that effect mesenchyme cells, which originated 
at an early stage in the annular region here alluded to, 
migrate over the surface of the umbilical vesicle and come 
to be situated between the layer of entoderm which forms 
its inner, and of splanchnic mesoderm which sooner (Pri- 
mates) or later (other mammals) forms the outer wall of 
this vesicle. Besides these lateral portions of the annular 
zone the hinder portion of it, situated diametrically opposite 
to the protochordal plate, has yet an important part to play 
in the formation of blood-vessels and blood. From it the 
vascularisation of the Haftstiel of the Primates is derived. 
From the distal end of this connective stalk vessels irradiate 
towards the whole inner surface of the diplotrophoblast (man 
and anthropomorphe) or only towards a restricted circular 
part of it (Tarsius). This vascularisation must phylogeneti- 
cally have preceded (as we will discuss later on) that which 
comes about by means of a free allantois. The thickened 
entoderm in this hinder part of the ring is especially marked 
in Manis. After a comparatively short time the annular 
entodermal region has ceased to be a focus of mesenchyme 
production; henceforth the increase of the vasifactive tissue 
is left to mitoses of the cells already constituting it. We may 
after careful consideration of all the mammalian preparations 
at our disposal all the more safely conclude to the existence 
of such migration of vasifactive cells if we consider that 
in other vertebrates (‘Teleosts) this very phenomenon has 
been actually observed in the live embryo by Wenckebach 
(86), Ziegler (’87), and others. 
In how far yet other tissue than blood-vessels and blood 
pathies with Kleinenberg’s (’86) drastic expression, “Ks giebt kein mittleres 
Keimblatt.”’ 
