556 PROFESSOR W. TURNER ON THE PLACENTATION OP THE APES. 
that aspect with a good lens, when the mouth of the artery can be seen to open 
obliquely on the inner surface of the decidua placentalis. In the Macacus, owing to 
the placental arteries being so much smaller in size, and not exhibiting a characteristic 
curling appearance, it was difficult to distinguish them from the arteries of supply of 
the decidua placentalis, and then- exact mode of communication with the interior of 
the placenta was not satisfactorily ascertained.* 
In the human placenta, the utero-placental veins form sinus-like dilatations, not 
only in the serotina occupying the placental area, but in the inter- cotyledonary dis- 
sepiments and at the margin of the placenta. These sinuses communicate with the 
interior of the placenta by cribriform apertures in their walls, through which it is 
not uncommon to see the placental villi project into the sinus. In the Macacus, the 
corresponding veins are not dilated into sinuses, but preserve, as already pointed 
out, their cylindrical tubular form. 
Both the utero-placental veins and the curling arteries of the human placenta 
possess a smooth inner surface, due to the presence of an endothelial lining. From 
the freedom of their communication with the intra-placental maternal blood spaces 
one would expect that a prolongation of this endothelial lining for a greater or less 
distance into the placenta would take place. Although such a prolongation is denied 
by many observers, yet I have occasionally seen an appearance of a lamella on the 
villi distinct from and external to their epithelial covering, which layer I have 
interpreted as derived from and representing the endothelial wall of the maternal 
vessels,! though without committing myself to the statement that it was universally 
present. Langhans has since given a description of an endothelial-like membrane 
on the placental surface of the decidua serotina in placentae in the fourth and seventh 
months, though he cannot speak with the same confidence of its existence in the 
placenta at the full time. Leopold, though unable to detect an endothelial covering 
on the villi, has seen it on the placental surface of the decidua between the place of 
attachment of the villi to that structure. From the free communication of the curling 
arteries and utero-placental veins with the interior of the placenta, and from the pro- 
longation of their endothelial lining for some distance at least into the organ as a wall 
for the intra-placental blood spaces, there can be no doubt that these spaces represent 
maternal blood vessels. They are, I believe, greatly dilated blood capillaries, the 
endothelial wall of which is in part preserved, though to a large extent it apparently 
disappears, so that the villi with their cellular covering lie in the maternal blood spaces, 
which have assumed a cavernous character. That the maternal blood capillaries, even 
at a very early period in the development of the decidua placentalis, become dilated 
into sinuses I have already pointed out (p. 548) in my description of the decidua 
serotina in human uteri from the third to the fifth week of gestation. 
I am unable to state if the endothelial lining of the uterine vessels in M. cynomolgus 
* See the Appendix for a more explicit statement on this point. 
t “ On the Placentation of the Sloths,” Trans. Roy. Soc. Edinburgh, vol. xxxii., p. 99, 1873. 
