86 A. A. W. HUBRECHT, 
perty that these vessels of the area vasculosa, ramifying over 
the yolk, as they did before the entoderm cells have become 
yolk-laden to that extent, carry this reserve-food towards the 
embryo. 
That the blood-vessels of the umbilical vesicle in mammals 
can attain a high calibre (Fig. 74) and that nevertheless 
the enclosed blood-cells have all the appearance in sections of 
not yet being freely suspended and movable I have noticed 
on earlier occasions (’89, ?90). I have since found this con- 
firmed in Tarsius, Tupaja, and others. And it would certainly 
not advocate against the haematopoietic significance of these 
blood-vessels, that also in Teleosts Wenckebach (’86) and 
Ziegler (87) have shown that a solid chord may gradually 
develop into a vessel with a wide lumen and with blood-cor- 
puscles that originally appeared as the inner core of the 
Anlage of the blood-vessel. 
Having started from the consideration of the small um- 
bilical vesicle of the Primates, we must now consider the case 
we find in the majority of mammals where the entoderm 
clothes the whole inner surface of the trophoblast at a very 
early period. So it does in Ungulates, but in these it 
becomes severed from the trophoblast comparatively soon 
again. This takes place when the mesoblast has developed and 
has become split into a somatic and a splanchnic layer. The 
splanchnic layer always remains very small as compared to 
the somatic in consequence of the enormous distension of the 
diplotrophoblast (Fig. 133). In other mammals, as in many 
Insectivores (Fig. 38), the separation between the umbilical 
vesicle and the diplotrophoblast is not so rapid, and sufficient 
time elapses before it comes about, so as to allow the vascular 
area that has in the meantime developed on the umbilical 
vesicle to become not only a centre of hematopoiesis, but 
now also a vehicle for a very appreciable exchange in a 
region which has been termed the omphaloidean placenta. 
I have elsewhere (’89, Pl. 18, fig. 32; Pl. 24, fig. 44) given a 
detailed description of this for the hedgehog and also for 
the shrew (794, figs. 7—11, 51, 83). 
