1 1 2 DE VEL OPMENT. 



ward folds commence first at the cephalic extremity, and subsequently at the caudal 

 end and sides, and deepen more and more, in consequence of the sinking of the 

 embryo into the blastodermic vesicle, until, gradually approaching, they meet one 

 another (Fig. 79, E 7). After they come in contact 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 fold, and the outer with the outer 

 (Fig. 79, F 7'). 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. 79, F and G, a) containing a clear fluid, the liquor amnii. 1 

 The other, the outer layer of the fold the false amnion lines the internal surface 

 of the original vitelline membrane. Between the two is an interval, which of 

 course communicates with the pleuro-peritoneal cavity. This it continues to do 

 until the body-walls of the embryo have grown up and coalesced at the umbilicus. 

 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, derived respectively from 

 the epiblast and from the parietal layer of the mesoblast. 



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 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 also. It contains about 1 

 per cent, of solid matter, chiefly albumen, with traces of urea, the latter possibly 

 derived from the urinary secretion of the foetus. 



The Chorion. We have seen that in the formation of the amnion we had 

 two layers formed out of a reduplication of the epiblast and outer layer of 

 the mesoblast : one the true amnion which surrounds the embryo and 

 encloses a cavity between it and the embryo the amniotic cavity ; and secondly, 

 the false amnion, which lies in apposition Avith the internal surface of the 

 vitelline membrane, and is continuous at its periphery with that part of the 

 original epiblast and somatopleural layer of the mesoblast which did not enter 

 into the formation of the area germinativa ; and that between these two layers 

 there is a space (which must not be confounded with the amniotic cavity) 

 which communicates with the pleuro-peritoneal space, and, according to Dalton, 

 contains a semifluid, gelatinous material. The chorion is formed out of the 

 vitelline membrane with the false amnion and its peripheral continuation with 

 the external layers of the blastoderm ; but the exact share which the three layers 

 take in its formation is at present uncertain. By some embryologists it is 

 believed that the vitelline membrane during the rapid growth of the ovum 

 becomes attenuated, and finally lost ; by others it is thought that it combines with 

 the other layers to form the chorion. But, whichever is true, at a very early 

 period of gestation cellular processes or fringes groAv outward from the external 

 surface of the chorion, and have been likened by Dalton to tufts of seaweed. 

 They are at first destitute of vessels, and are of simple cellular structure. These 

 fringes, or villi, as they subsequently become, cover at first the whole surface of 

 the chorion ; but as development progresses and the placenta, by which the 

 extent of the attachment of the ovum to the uterine walls is to be limited, is 

 about to be formed, the villi are not further developed over the rest of the 

 chorion, but are confined to that part only which is to form the foetal portion 

 of the placenta. They may, however, be recognized all over the chorion as 

 abortive processes during the whole of foetal gestation. 



1 The student should be careful not to confound this cavity with that formed between the true 

 and false amnion, which communicates with the pleuro-peritoneal cavity of the embryo. This 

 latter space ought with more propriety to be called the "amniotic cavity," since it is contained 

 between the layers of the amnion ; whereas the so-called amniotic cavity is not really between 

 the lavers of the amnion at all, but between the inner layer of the amnion and the body of the 

 embryo. 



