EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 147 
It has been noticed above (p. 103) that for such intensifica- 
tion of the osmotic exchanges the removal of the blastocyst 
out of the uterine lumen and its total inclusion within the 
mucosa by the formation of a so-called decidua reflexa or 
capsularis is very essential. The two most striking, and, at 
the same time, most perfect cases of this are presented (as 
was also already mentioned) by the hedgehog and by man. 
Still the two cases are in many respects different, but 
resemble in this respect that, whereas our primitive placental 
cases show a combination of phagocytosis and osmosis during 
& comparatively considerable portion of the period of preg- 
nancy, in the hedgehog and in man the phagocytosis is of 
great intensity in the beginning, but is followed by a second 
period in which the osmotic interchange is considerably 
perfected. This latter perfection is noticeable along two 
lines. First, the tissue separating the maternal and the 
embryonic blood is most considerably reduced, and while 
we yet noticed two epithelial and two endothelial layers 
between maternal and embryonic blood in many Ungulates, 
we see that in Insectivores and Primates it may become 
reduced to a simple membrane of maximal tenuity. We 
need not insist upon the very great difference this makes for 
rendering osmotic interchange ever so much more effective, 
and we are then no doubt justified in saying that the Primates 
and certain Insectivores represent a step in advance on our 
archaic type, Just as well as the Ungulates represented a 
retrograde step. 
A second improvement by which intensification of the 
osmotic processes is being brought about, concerns the extent 
to which embryonic vascular surface is brought in contact with 
maternal blood. Here, too, we see that man and to a some- 
what lesser extent the monkeys undoubtedly represent a maxi- 
mum of intensification of the osmotic process. The allantoic 
villi, exceedingly numerous and finely branched and covered 
only by the excessively thin layer of tissue above alluded to, 
present an all the more considerable surface for the osmotic 
processes because they are freely suspended in the maternal 
