STUDIES IN MAMMALIAN EMBRYOLOGY. 309 
upon the allantois surface. This may serve to explain how it 
is that in figs. 55 and 56 the villi, a. v., do not all of them appear 
to be connected with the allantois tissue. The section has 
made a small angle with the general direction of the villi and 
thus of most of them the stem has not been cut but only the 
_ top, which would here give the false impression of being 
detached from the allantois itself. That the villi again tend 
to ramify is visible in fig. 56,and at the same time that the 
extremity of the villus is attached to the maternal tissue by 
strands of trophoblastic cells, these strands being exceedingly 
short when the villus is long and reaches through the whole 
thickness of the trophoblast, as do most of them in the ripe 
placenta (cf. fig. 57). The villi in the ripe placenta are so 
intimately fused with the surrounding tissue that they can in 
many cases only be recognised (at least peripherally and in a 
section that is not rigidly perpendicular) by the embryonic 
blood-corpuscles that are contained in the vessels. These em- 
bryonic corpuscles, e. 6/, have the character of maternal leuco- 
cytes, much more than of the maternal red corpuscles m. b/. by 
which the surrounding trophoblastic spaces, sp., are gorged. 
About these trophoblastic spaces a few words will suffice in 
addition to what was already noticed when the earlier stages 
were described. They have since increased in size, but their 
mutual relation as well as their mode of communication with 
the maternal tissue has remained the same. The stratified 
layer retains its character as such even in the stage of fig. 55. 
In preparations of the later stages it was no longer very 
marked ; and it appears that its cellular elements, losing their 
elongated and fusiform shape, mingle with the outer layers of 
the subjacent trophoblast. Perhaps this fact may be looked 
upon as an argument to include the stratified layer with the 
embryonic instead of with the maternal elements (see p. 300). 
For myself I do not hold it as yet to be conclusive evidence 
either way and prefer to reserve my judgment as above formu- 
lated. At all events there is, if not any strict boundary line, 
at least a visible difference between the lower half of the 
placenta consisting of trophoblast and allantois and the upper 
