THE AMNION. 



tudinal column, divided into a number of square segments, the " protovertebrae." 

 (10) The splitting of the mesoblast, external to the protovertebrae, into two layers 

 the outer, called the " somatopleure," lined externally by the epiblast ; the inner, 

 called the "splanchnopleure," lined internally by the hypoblast, a space being left 

 between the two which forms the " pleuro-peritoneal cavity." (11) The curving of 

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

 canoe ; the walls being formed of all three layers of the blastodermic membrane 

 and the well of the canoe that is the intestinal cavity of the embryo, opening 

 into the cavity of the yolk-sac. (12) A portion of the yolk-sac lying in the body- 

 cavity of the embryo, and a portion outside it ; the two communicating by a duct, 

 the " omphalo-mesenteric " duct. The portion of the yolk-sac external to the body- 

 cavity is termed the umbilical vesicle, and provides nutrition to the embryo until 

 such time as the placenta is formed ; vessels, developed from the middle blasto- 

 dermic layer, ramifying over it, and gradually absorbing its contents. 1 



The next step toward a clear understanding of the development of the embryo 

 is to have a proper conception of the manner in which the membranes envelop- 

 ing the foetus are formed. 



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

 decidua. The tAvo former are developed from foetal structures, and are proper to 

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

 structures. 



The Amnion. The amnion is the membrane which immediately surrounds the 

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



False amnion or chorion. 



\ 



FIG. 86. 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. 1, 1890.) 

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



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. It is formed thus : At or near the extremities of 

 the incurved foetus that is to say, at the point of constriction of the blastodermic 

 membrane, where the portion which has undergone changes to form the body of 

 the embryo joins the part devoted to the formation of the umbilical vesicle an 

 inflection of the epiblast and outer layer of the mesoblast, which have become 

 separated from the inner layer of the mesoblast and hypoblast by the formation of 

 the pleuro-peritoneal cavity, takes place (Fig. 79, D 7). These inflections or back- 



1 According to Professor John A. Ryder, it is " very doubtful if any considerable amount of 

 nutriment is supplied to the embryo from the yolk-sac at any time." 



