360 ALA Nis HUBRECH'. 
which are centrifugal, have not to force their way. On the 
contrary, the allantoic tissue fits between and extends itself 
around these trophic papilla, ensheathing them more and more 
intimately as the placenta further ripens. 
The gradual appearance of these papill on the inner concave 
surface of the trophosphere was fully noticed above for the 
hedgehog. It can be very accurately followed for the bat if we 
successively compare the different developmental stages of the 
trophocalyx in Frommel’s figs. 11 to 16, 20 and 21. Outside 
this embryonic trophoblast, which in the ripe placenta is still 
more intimately fused with the allantoic tissue, Frommel’s 
fig. 21 shows us a layer which he marks #. L. It is the evident 
counterpart of what I have called in the hedgehog the tropho- 
spongia, i.e. that layer of the trophosphere which has arisen 
out of a proliferation of the endothelium of maternal blood- 
vessels, which have themselves disappeared by this process. It 
is more elaborately figured on Frommel’s Pl. x. 
The rather compact cell-material is perforated by irregular 
lacune partly derived from the lumen of the blood-vessels that 
give rise to this particular layer. Frommel calls this layer the 
“epithelial layer”’ of the placenta, but when we go back to his 
earlier figures above cited we find that it develops out of what 
he first calls the “ Gefiissschicht.” This ‘‘ Gefissschicht ” 
(vascular layer) from the beginning immediately surrounds the 
layer D.S. (the embryonic trophoblast) ; it is, however, less con- 
spicuous in the earlier stages (Frommel, l.c., p. 88). To facili- 
tate comparison with what I have above described for the 
hedgehog’s trophospongia, and at the same time to show the 
reason by which we are authorised to identify these layers in 
the two species, I will here insert a citation from Frommel’s 
text. He says (l.c., p. 33), speaking of the vessels of the 
Gefassschicht, at the time their metamorphosis into a tropho- 
spongia is imminent: 
«The endothelium (of these vessels) increases in height, 
the cells increase in size, and the transverse section of such a 
vessel is not unlike that of a gland. But very soon this aspect 
changes, as the vascular lining commences to proliferate very 
