144 A. A. W. HUBRECHT. 
cyst fit. In the case of the polycotyledonary placentation, 
osmotic and phagocytic absorption is yet combined, in that 
of the diffuse placentation of the horse it would seem as if 
the osmotic interchange between the maternal and embryonic 
blood (which takes place all over the extensive surface where 
the villi interlock in the crypts) has by far superseded 
phagocytic nutrition. There is an intact double epithelial 
layer, one maternal, one trophoblastic, that everywhere 
separates the two blood-fluids; nevertheless the considerable 
surface over which the two circulatory systems are in such 
very close proximity seems to make up for what is lost in 
exiguity of the separating membranes. And so the placentary 
arrangements, as we find them in the horse, appear to me as 
an extreme state of specialisation of what in Carnivores, some 
Insectivores, and in Didelphia was a more primitive but a more 
complicated arrangement. The fixation of the blastocyst by 
means of adhesive and phagocytotic properties of the tropho- 
blast cells seems to have been reduced to a minimum; the 
phagocytosis, which was certainly more active in the Artio- 
dactyla, where also the fixation by means of the cotyledons 
was somewhat more firm, is in no way prominent in the horse, 
but the possibility of osmotic processes between large sur- 
faces of maternal and foetal vascularised tissue has reached 
a higher degree of development. 
The polycotyledonary arrangement has thus retained more 
hereditary points in common with the primitive placentation 
described above, than the diffuse. Tragulus meminna has 
already been cited (p. 113) in support of this. Also the less 
considerable degree of specialisation, which we find in the 
skeletal parts of the limbs, would correspond with the smaller 
amount of placentary specialisation. The sequence in placental 
complication would thus have to be reversed; it is not the 
polycotyledonary arrangement that represents an advance as 
compared to the diffuse, but it is the diffuse that should be 
looked upon as the last rung of a ladder of simplification 
which the placental processes have undergone in the Ungu- 
lates, starting from the arrangements above alluded to, which, 
