EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 143 
so much more extensive, and offers more copious opportunities 
for the absorption of nutritive material either out of the 
uterine lumen or, more indirectly, out of the vascularised 
mucosal surface, be this provided with an epithelium or 
deprived of it. 
This state of things we find realised in Ungulata, in Cetacea, 
and in certain Hdentates. Both for the sheep and the pig 
Bonnet and Keibel, and earlier authors before them, have 
made us acquainted with a most considerable growth in size 
of the sometimes even tubular blastocyst (Figs. 155 and 154), 
on the surface of which the embryonic shield only occupies a 
hardly visible space (total length of blastocyst 21 cm., breadth 
1} mm.; length of corresponding embryonic shield 1 mm.). 
This considerable surface increase, which is also found in the 
Equide and other Ungulates which have hitherto been ranked 
as representatives of diffuse and polycotyledonary placenta- 
tion, is thus seen to go parallel to a certain extent to a not 
inconsiderable increase in the size of the adult animal, with a 
corresponding increase in the size of the, generally bicornuate, 
uterus. 
The conditions in which we find the free allantois in these 
Ungulates show that the considerable enlargement of the 
blastocyst has only commenced after a free allantois had 
already been evolved out of the earlier arrangements. Before 
the allantois has spread out against the inner surface of the 
diplotrophoblast, the outer trophoblastic investment has full 
occasion to be very active in elaborating and transporting 
the detritus in the uterine lumen, which has been termed 
“uterine milk,” inside the cavity of the blastocyst. After 
the allantoic vascularisation of the diplotrophoblast has come 
about the latter becomes applied against the maternal surface, 
where at numerous, but independent, spots (so-called carun- 
cule), the tissue has been prepared by the formation of 
so-called cotyledons, into which fit groups of allautoic villi. 
In other Ungulates no cotyledons are present, but the 
maternal surface is thrown into a dense network of folds and 
crypts, into which corresponding folds or villi of the blasto- 
