No. 3.] UTERUS AND EMBRYO. 385 
slight condensation, mes, of the connective tissue to form, as it 
were, an envelope. This structure has been regarded by Ahl- 
feld and others as the persistent yolk sack. I think the correct 
interpretation was first suggested by Kolliker. 
§ 14. Amnion. — The tissues of the amnion do not progress 
beyond an early embryonic stage; the ectoderm remaining at 
the one-layered stage, the mesoderm preserving much of the 
primitive matrix. Emery (Arch. [tal. Biol., III., 37) has directed 
attention to the primitive homogeneous matrix of the vertebrate 
mesoderm, and especially to the separate sub-epidermal layer of 
the embryo, which contains no cells at first. In the human 
Cut 8.— Two sections of the placental amnion: A, from an embryo of the eighth 
month; B, at term; ec¢, ectoderm; #zes, mesoderm; a, layer of mesodermic cells. 
X 340. 
amnion there is a non-cellular layer under the epithelium, as is 
well shown in Cut 8, A and B. Sometimes this layer is in- 
vaded to a certain extent by connective tissue cells, B ; in other 
cases the portion of the matrix towards the chorion acquires a 
fibrillar character, A, as if partially resorbed, but in no case 
have I seen the matrix entirely altered from its primitive char- 
acter. The cells of the mesoderm lie in lacunz ; they are flat- 
tened in the plane parallel to the surface, and hence in vertical 
sections, Cut 8, appear more or less fusiform. They present no 
special features, so far as I have observed, to distinguish them 
