3886 A. A. W. HUBRECHT, 
with that of His (from which it differs in the points that 
concern the formation of the amnion, as above indicated on 
p. 374), derives a further support from the fact that in the 
hedgehog (and perhaps many other mammals) the somatic meso- 
blast is at the time of its first appearance composed of cells 
that are relatively ever so much more bulky than they are later 
on (vide the cells m. som. of figs. 43 and 52). Thus the 
vascularisation of this layer at an early period is more 
easily understood. The physiological explanation may be as 
follows: Once the embryonic circulation having found the 
shortest route towards the trophoblast by the way of the ventral 
stalk, trophoblastic lacunze with their profusion of maternal 
blood, which have been there from the very earliest periods of 
development, are exquisitely situated for rendering this new 
adaptation highly advantageous. And while in the ancestral 
forms of the Primates both yolk-sac and allantois largely drew 
upon the trophoblastic source in the way we have above very 
fully described, these embryonic organs come to be dispensed 
with to a very great extent in their more highly developed 
descendants, who come to utilise that trophoblastic source 
along a more direct, a shorter, and an earlier established 
route. 
It is quite intelligible that the later phases in which the 
more extensive surface connection between embryonic and 
maternal circulation comes to be replaced by a discoid arrange- 
ment, should follow similar paths both in the hedgebog and in 
man, even though in the one case it is part of the chorionic cir- 
culation (chorion leve) in the other omphaloidean vascular 
tissue that is after a time suppressed. Intelligible because in 
both cases the embryo, surrounded in its earliest stages by a 
decidua reflexa, expands this during its growth, thus dimin- 
ishing the nutritive significance of the reflexa. 
The embryo is more and more restricted, for a thorough 
intercourse with the trophoblastic blood recesses to the region 
at the bottom of the depression in which it has become encap- 
suled. It is there that in both cases the discoid placenta then 
takes its origin, | 
