1160 



EMBRYOLOGY. 



to form the " somatopleure " and " splanclmopleure," the space between the 

 two constituting the " coelum " or " pleuro-peritoneal cavity." (11) The curving 

 of the embryo on itself, both longitudinally and laterally, so as to be comparable 

 to a canoe, part of the blastodermic vesicle being enclosed within the embryo to 

 form the "primitive alimentary tube," part being left outside as the "yolk-sac," 

 the two communicating by a duct, the " omphalo-mesenteric " duct. The yolk-sac 

 provides nutrition to the embryo through the omphalo-mesenteric vessels until such 

 time as the placenta is formed. 



FORMATION OF MEMBRANES. 



In order to have a clear understanding of the manner in which the embryo is 

 developed, it is necessary at this stage to describe the development of the foetal 

 membranes. 



The membranes investing the foetus are the amnion, the chorion, and the 

 decidua. The first two are developed from foetal structures, and are proper to 

 the foetus ; the last is formed in the uterus, and is derived from the maternal 

 structures. 



The Amnion. The amnion is the innermost of the membranes which surround 

 the embryo. It is at first of small size, but increases considerably toward the 

 middle of pregnancy, as the foetus acquires the power of independent movement. 

 It exists only in reptiles, birds, and mammals, which are hence called " Amniota," 

 but is absent in amphibia and fishes. In man, monkeys, and some of the bats, 

 the primitive amniotic cavity, already described on page 1154, persists. In reptiles, 

 birds, and certain mammals the amnion is formed in the following manner. At or 



False amnion or chorion 



van of^\ 



chorioji ~"~*-y' 



FIG. 705. Diagram of a transverse section of a mammalian embryo, showing the mode of formation of the 

 amnion. The amniotic folds have nearly united in the middle line. (From Quain's Anatomy, vol. i. pt. i. 

 1890.) Epiblast, blue ; mesoblast, red ; hypoblast and notochord, black. 



near the extremities of the incurved foetus that is to say, at the point of constric- 

 tion of the blastodermic vesicle where the primitive alimentary canal of the embryo 

 joins the yolk-sac a reflection or folding backward of the somatopleure, which 

 has become separated from the splanchnopleure by the formation of the pleuro- 

 peritoneal cavity, takes place (Fig. 702, 2, 3). This fold commences first at the 

 cephalic extremity, and subsequently at the caudal end and sides, and deepens 

 more and more, in consequence of the sinking of the embryo into the blasto- 

 dermic vesicle, until, gradually approaching, the different parts meet on the 

 dorsal aspect of the embryo (Figs. 702, 10, and 705). After they come in contact 



