EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 181 
3. The Phylogeny of the Placenta. 
Although it may yet be too early to venture upon the 
attempt of sketching a phylogeny of the placenta, different 
from that in which the diffuse placenta is looked upon as the 
starting-point such as we find it generally accepted in text- 
books, still I may be allowed to bring forward certain con- 
siderations which ought to be kept sight of whenever that 
sketch is drawn up. 
In the first place the old and catching comparison between 
the very early villiferous state of the human blastocyst, which 
in the phase of, for example, the so-called Reichert’s ovum 
was said to pass ontogenetically through a diffuse phase to 
which the discoid stage only succeeded later, ought to be 
definitely got rid of, as I have already suggested long ago 
(89, p. 339). The fact is that this so-called villiferous stage 
of the human ovum does not resemble the diffuse placenta at 
all because (1) Reichert’s ovum is incomplete, and if complete 
would not have a villiferous but a sponge-like aspect, the 
so-called villi being actually transversely connected super- 
ficially (cf. Figs. 36—40) ; (2) in consequence of the presence 
of a decidua reflexa (capsularis) it is not freely suspended in 
the uterine cavity as are the blastocysts which show the so- 
called diffuse placentation ; (3) there are no maternal crypts 
clothed with epithelium into which the villi fit, but these are 
directly bathed by maternal blood. 
Once this comparison being discarded, we ought to look 
the question in the face whether the diffuse placentation as 
we find it in the horse, the pig, and the lemurs, does really 
represent the first step on the road that finally leads to the 
very complicate placental arrangement of man and other 
mammals. ‘The three examples just named are in themselves 
sufficient to arouse a certain a priori suspicion. We could 
hardly expect that the most primitive type of placentation 
should be retained in an animal that is so eminently specia- 
lised as the horse. Nor in an order such as the Lemurs that 
