554 
PROFESSOR W. TURNER ON THE PLACENTATION OF THE APES. 
however, to this view, it may be stated that the chorionic epithelium apparently 
degenerates and disappears in the later months of pregnancy, whilst this layer of cells 
persists throughout the whole period of gestation. The late Professor Goodsir, in 
his memoir “On the Structure of the Human Placenta,” * described them as derived 
from the decidua, and as belonging therefore to the maternal part of the placenta. 
The observations made from time to time, and quite independently of each other, by 
Signor Ercolani and by myself on the minute structure of the placenta in numerous 
mammals, have proved that there is interposed between the maternal blood and the 
foetal villi a layer of cells derived from the uterine mucous membrane, and belonging 
therefore to the maternal placenta. The cellular covering of the villi in the human 
placenta and in that of the Macacus has a similar relation on the one hand to the 
maternal blood, and on the other to the tissue of the villus and to the capillary 
terminations of the umbilical vessels, which the epithelial cells of the maternal part or 
the placenta in the diffused, cotyledonary, and zonary forms of placenta have to the 
maternal and foetal blood vessels. On this ground, therefore, they might be regarded 
as homologous with each other. 
In order, however, to prove the derivation of the cellular covering of the human villi 
from the decidua, it would be necessary to trace a prolongation of the decidua around 
them. In the more advanced of the two early human gravid uteri I have already 
described (p. 54G), I found that the villi were adherent both to the decidua serotina 
and to the inner surface of the decidua reflexa, from both of which delicate pro- 
longations passed between the villi, so that the fixing and interblending of the two 
structures seemed to be due to a coincident growth of the villi and of the decidua. 
It is probable, therefore, that at this early stage of placental development the v illi 
may become ensheatlied by the cell structures of the decidua. In the placenta of the 
fifth month, as I have described at p. 550, the interlocking of the foetal villi with trabe- 
cular prolongations of the decidua was distinctly recognised. Even in the fully- 
formed placenta the attachment of the terminal parts of the villi to the placental 
surface of the decidua shows how intimate is the relation of the one structure to 
the other. I have no observations to offer on the early stages of development of the 
placenta in Macaous, but from the similarity in appearance and arrangement of the 
cellular covering of the villi to that in the human placenta there can be no doubt that 
it has a similar origin. Should the more superficial layers of the sub-chorionic cells be 
derived from the decidua, then an additional argument may be urged in support of 
the decidual origin of the cellular covering of the villi. 
Both in the Macacus and in the human female the stems of the villi arose from the 
placental surface of the chorion, separated from each other by intervals which in the 
Macacus were from one-tenth to two-tenths of an inch, and in the human placenta 
from two-tentlis to three- tenths. In the Macacus, the stems, though giving off nume- 
* ‘ Anatomical and Pathological Observations,’ Edinburgh, 1845. ‘Anatomical Memoirs,’ Edinburgh, 
1868. 
