148 A. A. W. HUBRECHT. 
blood and thus bathed on all sides ; whereas, for example, in 
Tarsius, in the hedgehog, and in other Insectivores, although 
there is only a very thin membrane separating maternal and 
foetal blood, still the section shows a very fine sponge-work 
of the finest membranous structures between which the allan- 
toic villi are densely distributed. As they are, however, not 
freely suspended, but stretched between and supported by 
the meshwork here alluded to, the total surface available for 
osmotic interchange must necessarily be relatively less. 
It would seem as if, in the human placenta, there is still 
left a certain margin for phagocytotic processes, brought 
about by the so-called “syncytial cells,’ which are present 
here and there on the villi, and are nothing but remnants of 
a plasmoditrophoblast (cf. Bryce and Teacher, ’08). An im- 
portant fact which was mentioned on p. 111 is the discovery 
by Assheton (’06) of the early placentary stages ina primitive 
Ungulate asis Hyrax. It adds considerably to the probability 
that the simplification which was above suggested as having 
occurred in the phylogeny of the Ungulate placenta 1s, indeed, 
the actual explanation of the phenomena such as we notice 
them. 
4, Summary of Chapters IV and V. 
In concluding this and the preceding chapter I wish to 
emphasise that we have established an undeniable activity in 
the trophoblast of monodelphian and of didelphian mammals 
preceding and accompanying placentation, and that we have 
at the same time shown that those orders where such activity 
was insignificant or absent (Lemurs, certain Hdentates and 
many Ungulates) must in this respect be looked upon as 
having been secondarily modified by various circumstances. 
Direct indications of this retrograde process are not wanting. 
This being the case and the well-known and apparently 
natural starting-point which the so-called diffuse placeuta 
offered us for establishing the phylogeny of placentation 
having thus broken down, we have attempted to establish 
