EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 127 
maternal blood circulates freely; later the nutritive processes 
are more concentrated in the placenta. 
c. Primates.—Of Primates I will just yet touch upon the 
placenta of Tarsius and man. ‘The first is the result of a 
limited trophoblastic proliferation simultaneous with a tropho- 
spongian process by which interglandular mucosal tissue 
prepares a surface with which the trophoblastic proliferation 
very soon forms a most intricate concrescence, in which 
maternal blood freely circulates, and which attains to com- 
paratively considerable thickness before embryonic vessels 
have yet become ensheathed between the trophoblastic 
proliferations (Hb., ’99, Pl. 11, fig. 67). Soon after this 
latter process begins, further increase only occurs in this 
trophoblast and its enclosed embryonic vessels, the tropho- 
spongia remaining active only in the zone where the placenta 
will separate from the maternal tissues, this zone being in the 
latter half of pregnancy only a stalk (Fig. 147) through which 
arteries and veins have access to the placental blood-spaces. 
Such a stalked condition of the placenta is also character- 
istic (Fig. 158) for certain Rodents (mouse), and to a certain 
extent for Sorex, whereas in the squirrel, the hedgehog, in 
man, in Galeopithecus, and others the discoid placenta may 
be termed sessile over its whole proximal surface. Hzmato- 
poietic processes occurring in Tarsius during placentation 
have been noticed by me elsewhere (799, p. 368, Pl. 14). 
The placenta of man has already been alluded to on p. 101. 
There is no doubt that in it trophoblastic elements play quite 
an overwhelming part, much more so than was recognised by 
earlier observers (Figs. 142,143). Thanks to the investigations 
of von Heukelom (’98), Peters (99), and Bryce and Teacher 
(08) we have now also become acquainted with part of 
the trophospongian arrangements in man, and a prediction 
of mine (’89) that the early, then as yet unknown stages 
of the human placentation, would offer close resemblance 
to what we find in the hedgehog has been fully confirmed by 
the authors alluded to. 
One of the most notable differences between the placenta 
