FORMATION OF MEMBRANES. 



1161 



they fuse together, and the septum between them disappears ; so that the 

 inner layer of the cephalic fold becomes continuous with the inner layer of 

 the caudal and lateral folds, and the outer with the outer. Thus we have two 

 membranes, one formed by the inner layer of the fold the true amnion which 

 encloses a space over the back of the embryo the amniotic cavity (Fig. 702, 4, 5) 

 containing a clear fluid, the liquor atnnii. The other, the outer layer of the fold 

 the false amnion lines the internal surface of the original zona pellucida. 

 Between the two is an interval, which of course communicates with the pleuro- 

 peritoneal cavity until the body-walls of the embryo have coalesced at the umbili- 

 cus. Then the amniotic fold is carried downward, and encloses the umbilical 

 cord, by which the foetus is attached to the placenta. The true amnion 

 or, as it is usually called, the amnion is formed of two layers, inner and outer, 

 derived respectively from the epiblast and from the parietal layer of the meso- 

 blast. 



The amnion is at first in close contact with the surface of the body of the 

 embryo, but about the fourth or fifth week fluid begins to accumulate, and thus 

 separates the two. The quantity of fluid steadily increases up to about the sixth 

 month of pregnancy, after which it diminishes somewhat. The use of the liquor 

 amnii is believed to be chiefly to allow of the movements of the foetus in the later 

 stages of pregnancy, though it no doubt serves other purposes. It contains about 

 1 per cent, of solid matter, chiefly albumen, with traces of urea, the latter prob- 

 ably derived from the urinary secretion of the foetus. 



FIG. 706. Transverse section through the dorsal region of an embryo chick, end of third day. (From Foster 

 and Balfour.) Am. Amnion. mp. Muscle-plate, rv. Cardinal vein. Ao. Dorsal aorta, at the point where its 

 two roots begin to join. Ch. Notochord. Wd. Wolfflan duct. Wb. Commencement of formation of Wolffian 

 body. ep. Epiblast. So. Somatopleure. Hy. Hypoblast. The section passes through the place where the ali- 

 mentary canal (hy) communicates with the yolk-sac. Sp. Splanchnopleure. 



The Chorion. The chorion takes its origin, as has already been seen (page 1154), 

 from the external covering of the blastodermic, vesicle the cells of the decidua or 

 uterine mucous membrane contributing no elements to it. From its outer surface 

 numerous finger-like processes, termed the villi of the chorion, project. These 

 increase rapidly in size and at the same time undergo great ramification ; hence 

 they were likened by Dalton to tufts of seaweed (Figs. 705 and 708). They invade 



