EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 105 
ably on one side, viz., the surface that is contiguous with 
the uterus lumen. This thinning out goes parallel to, and is 
largely caused by, the increase in size of the growing blasto- 
cyst. The natural consequence is, that also the trophoblastic 
investment of the blastocyst is simultaneously considerably 
flattened as far as it is covered by the decidua capsularis. 
The maternal investment retains its thickness only along a 
saucer-shape zone furthest from the original uterus Jumen. 
It is in this part of the trophoblastic proliferation that 
the allantoidean placentation comes about and reaches its 
maximal development; a saucer- or disc-shaped placenta, 
which I have fully described in a former publication (’89), is 
the final outcome. 
b. Primates.—The investigations of Peters (’99), Sie- 
genbeek van Heukelom (98), Selenka (’00), Strahl (02, 
’04), Spee (96), Kollmann (’92), and, quite lately, Bryce and 
Teacher (08) have revealed to us that the early tropho- 
blast of man and the anthropoid monkeys is very similar 
(Figs. 59, 40, 145, 144) in its general line of development to 
that of the hedgehog. The very early stages and the exact 
way in which the human blastocyst comes to be imbedded 
in the maternal mucosa are, however, yet insufficiently 
known. Moreover, as we will see in Chapter VI, the pla- 
centary lacune are more spacious and the villi partially 
free and floatingly suspended in the maternal blood that 
circulates in these trophoblastic lacune (Figs. 141, 144). 
In the catarrhine monkeys, which differ from the Anthro- 
pomorphee and from man by the absence of a decidua reflexa 
(capsularis), the trophoblastic proliferation is no longer 
equally distributed over the whole surface, but is restricted 
to two regions opposite to each other, and corresponding to 
what wiil later become the dorsal and the ventral placenta 
(Figs. 132, 142). There is thus a circular zone along which 
there is hardly any proliferation of the trophoblast. I hold 
this arrangement to be secondarily derived from the complete 
enclosure present in man and the anthropoids. Again in 
Tarsius, which I have also classed with the Primates on the 
