EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 135 
trophoblast cells, who again pass the food thus obtained 
either to the embryonic blood-corpuscles or into the cavities 
inside the trophoblast, be this umbilical sac or extra-embryonic 
ccelom. 
The details of these physiological and complicated nutritory 
processes are still out of our grasp, and nevertheless they have 
undoubtedly very important significance by the side of more 
simple osmotic phenomena. Bonnet recognises (’02, p.489) that 
the actual feeding of the trophoblast cells on albuminiferous 
symplasmata, on fat, and on the morphotic substances of the 
maternal blood, as it takes place under our eyes, considerably 
facilitates our understanding how the proteids which diffund 
with so much difficulty pass from the mother into the embryo. 
Similarly the supply of iron in the mammals which have no 
ferruginous yolk to fall back upon, and which nevertheless 
must take place in utero, becomes explicable in this manner. 
I feel confident that these researches of Bonnet and others 
on the nutritive resources of the Carnivores are of the highest 
importance for a full understanding of the placentation 
process, of which the starting-point would then be the 
combination of adhesive and of phagocytic properties in the 
trophoblast cells. The same investigator, Bonnet, has in an 
anterior publication made us acquainted with the presence in 
the sheep’s uterus of a substance which he has named 
“uterine milk.” 
It is in reality the product of catalytic processes of the 
same sort as those that were described above, and it differs 
from the material produced in the Carnivores only in this 
respect, that it has been set free into the uterus lumen. A 
transition stage between the two is, perhaps, that case of 
the Tragulus embryo (another Uugulate already cited on 
p- 115), in which formed elements were seen to pass out 
of the maternal connective tissue, through a layer of tropho- 
blast into the embryonic tissues. At all events, there is an a 
priori probability that the arrangement in which organic 
detritus in the uterine lumen is being absorbed by the 
embryonic trophoblast is a later development from that in 
