STUDIES IN MAMMALIAN EMBRYOLOGY. 343 
in man, but the type is foreshadowed in the hedgehog. It is 
often more complicated than that of the allantoic villi repre- 
sented in figs. 55 and 56. Moreover, the attachment of these 
villi to the trophospongia (which may be directly compared to 
what is the serotina in man), as well as the communication of 
the large blood-cavities in the trophospongia with the lacunz 
that bathe the villi, &c., constitute new points of comparison 
or even of resemblance between the human and the Erinacean 
placenta. In the latter the tissue which forms the convex 
surface of the after-birth, and which is composed partly of the 
proliferated decidual tissue (in its fibrillar phase), partly of the 
trophospongia, may be compared to what is called Winkler’s 
*“‘ Basalplatte ” in the human subject (cf. p. 333). 
We have now seen that the trophosphere and the maternal 
blood form together a very effective nutritory arrangement for 
the embryo; that embryonic and maternal elements are fused 
together so completely in the trophosphere as to make its 
double origin a point of difficult, though feasible and instruc- 
tive, comparative investigation ; and that by these arrangements 
the hedgehog’s blastocyst may be said to be from its very 
earliest stages fixed in a highly nutritive enclosure resembling 
that of the human embryo. 
The villi of the earliest trophoblast are diffused over the 
whole of the blastocyst; they are the pathfinders for the vas- 
cular outgrowths of both yolk-sac and allantois, and at the 
same time they develop into a reservoir of maternal blood. 
The vascular outgrowths of yolk-sac and allantois thus on no 
occasion penetrate or grow into maternal tissue; it is em- 
bryonic tissue that carries the maternal blood towards them. 
With regard to the tissues constituting the trophosphere, it 
may well be asked whether the great similarity in appearance 
and arrangement between the cellular elements of the tropho- 
spongia and of the trophoblast is not also highly conducive to 
that perfect fusion which is so effectual for the discharge of 
their important functions? At the same time the question 
the ‘Journal of Morphology,’ vol. ii, 1889, in his article, “Uterus and 
Embryo,” is especially instructive in this respect. 
VOL. XXX, PART 3,—NEW SER. Z 
