No. 3.] UTERUS AND EMBRYO. 395 
the degenerated epithelium of the chorion and the upper layers 
of the decidua, as shown in Cut 25, wv, the epithelium and 
decidua being so fused at this point that it is impossible to 
determine any line of demarcation between them. 
The chorion, being a portion of the somatopleure, consists, of 
course, of two primary layers, the mesoderm and ectoderm. 
During the second half of the first month, the earliest period 
concerning which we have any accurate knowledge, the meso- 
derm is already a vascular layer of considerable thickness 
(Cuts 16 and 18, mes), and the epithelium (ectoderm) has two 
layers of cells (Cut 18, aand 4) ; of which the outer is the darker 
in specimens stained with osmic acid, carmine, cochineal, or 
haematoxyline, and has also smaller and more granular nuclei. 
The same distinction exists in the two-layered stage of the ecto- 
derm of the umbilical cord (Cut 3), and of the foetal skin. 
Hitherto most authors have entirely overlooked the inner layer 
at early stages. It was first clearly recognized by Langhans, 
who directed attention to it in a special memoir, 111, he having 
already described its later history, 110. In some earlier writers 
are allusions to the layer. Kastschenko, in his paper on the 
chorionic epithelium, has also described it, although he has not 
followed its history very far. The interpretation to be offered 
seems to me clearly to be, that the chorionic epithelium advan- 
ces in its differentiation to a stage equivalent to the two-layered 
stage of the epidermis and there stops ; whatever further change 
occurs is degenerative. 
The two primitive layers of the chorionic epithelium have 
been more or less clearly observed at later stages by several 
anatomists, and have been variously interpreted. Ercolani and 
Turner regard them as absolutely distinct, assigning the deep 
layer to the chorion as its true and only epithelium, and the 
outer layer to the uterus, thus enabling themselves to conceive 
the villi as covered by a maternal as well as a foetal epithelium, 
so that maternal blood found between the villi is still within the 
maternal tissue. After accepting the outer layer as maternal, 
the question as to its origin still remained. Some authors 
affirmed it to be the uterine epithelium, others to be the lining 
of expanded uterine blood sinuses. So far as I am aware, no 
one has made observations to show by the developmental history 
of the layer, that one or the other of the last mentioned hy- 
