No. 3.] UTERUS AND EMBRYO. 435 
knowledge of the nutrition of the embryo. We know positively 
scarcely more than that the maternal and fcetal circulations are 
brought very close together in the placenta. We infer that 
there must be a transfer of nutritive material from one blood to 
the other. As to wat material is transferred and how, we have 
only theories, but of them an abundance. Under these circum- 
stances, the best beginning is undoubtedly a frank acknowledg- 
ment of our ignorance. 
§ 22. Summary.— The following paragraphs attempt to give 
the more important of the conclusions reached in the second 
part of this paper. 
§ 13. The umbilical cord is not covered by the amnion, but 
by an extension of the foetal epidermis. Its caelomatic cavity 
is completely obliterated during the third month, and a little 
later the stalk of the yolk sack is resorbed. The allantoic epi- 
thelium persists as a tube or cord of cells for a long period. 
The blood-vessels have specialized walls derived from the sur- 
rounding mesoderm, but have no true adventitia. Connective 
tissue fibres begin to develop during the third month. 
§ 14. The amnion is covered by a single layer of ectodermal 
cells, which are connected by conspicuous intercellular bridges. 
It has no true stomata. -Its mesoderm consists of anastomosing 
cells, with a dense matrix; it is imperfectly divided into three 
strata, of which that next the ectoderm is without cells, that 
furthest from the ectoderm is often of a loose texture. 
§ 15. The chorion consists of two layers, mesoderm and ecto- 
derm, both of which are present over all parts of the chorion 
throughout the entire period of pregnancy. The mesoderm has 
at first a dense colorable matrix, with cells, which color very 
slightly. During the second month the matrix loses its coloring 
property, and subsequently the cells acquire a greater affinity 
for coloring-matters ; the matrix assumes a fibrous appearance, 
and ultimately in the region of the chorion frondosum connec- 
tive tissue fibrils appear in it, most numerously next the ecto- 
derm, so that the mesoderm is differentiated there into an outer 
fibrillar layer and an inner and thicker stroma layer. The ecto- 
derm during the first month divides into two strata, an outer 
dense protoplasmic layer and an inner less dense cellular layer. 
In the latter part of pregnancy the whole ectoderm of the chorion 
lave has acquired the character of the cellular layer, except close 
