THE FCETUS. 1019 



ing at some points fat-granules. 2. A chorial stroma of embiyonic connective 

 tissue — fibres, stellate cells, and round and migratory cells in the midst of a 

 large quantity of albuminous fluid. Dastre has observed in this layer opaque 

 plates — chorial plates — composed of irregular particles of tribasic phosphate of 

 lime, which ultimately serves for the ossification of the foetal cartilages. The 

 chorial deposit in Solipeds has the appearance of a network, the meshes and 

 spaces in which are badly defined. 3. A layer of mucous connective tissue that 

 unites it to the allantois. 



From this description, it will be seen that the chorion does not play a merely 

 mechanical part in protecting the foetus and supporting the placental blood- 

 vessels, but that it holds in reserve the materials that will serve for rapid 

 nutritive changes at a given moment. 



2. The Amxion (Figs. 556, A ; 557, C). 



The second sac enveloping the foetus — the amnion — floates freely in the interior 

 of the chorion, to which it is only united at one point through the medium of 

 the umbilical cord. It contains the young creature, which is also attached to 

 its inner face by the vessels of the cord. It has the shape of an ovoid pouch 

 depressed around the umbilical vessels, around which it forms a sheath as it goes 

 to be confounded with the skin of the foetus. It has thin transparent walls. 



Its external face is covered by the inner layer of the allantois, to which it 

 adheres slightly. A large number of flexuous vessels, enveloped by a thick layer 

 of mucous tissue, course over this surface. The internal face is perfectly smooth, 

 and is applied more or less directly to the skin of the foetus. It exhales a fluid 

 in which the latter floats — the liquor amnii. 



Structure. — As the amnion is derived, hke the chorion, from the somato- 

 pleure, it is not surprising to find in it three superposed layers : 1. A very thin 

 connective-tissue membrane that adheres to the allantois. 2. A proper membrane 

 — also of connective tissue — but containing some muscular fibres, which explains 

 the contration of the amnion observed in the chick. 3. An epithelial lamina 

 lining the latter. There are sometimes met with, at certain points on the inner 

 surface of the amnion, small, white, opaque masses, composed of glycogenic ceUs, 

 which become brown when treated with iodine. 



Liquor amnii. — Enclosed with the foetus in the cavity of the amnion, this 

 fluid is more or less abundant, according to the period of gestation ; its relative 

 quantity being always less as the foetus is advanced in development. At an 

 early period it is somewhat milky in appearance, but later it assumes a citrine or 

 slightly reddish tint. It has a salt taste, and contains 99 per cent, of water, 

 with albumen and salts, the principal of which are chloride of sodium and the 

 sulphate and phosphate of lime. 



3. The Allantois (Fig. 556). 



The allantois is a membrane that covers the inner face of the chorion, and 

 is folded around the insertion of the umbilical cord, to spread itself over the 

 whole external surface of the amnion. It thus transforms the chorial sac into 

 a kind of serous cavity, in which the amnios is enclosed as a viscus. 



The inner, or amniotic lamina, is attached to the amnion " so shghtly, that 

 dissection, and especially insufflation, easily destroys its adhesion. When the 

 second of these measures is resorted to, in order to separate the two membranes. 



