THE AMXION, 703 



highest degree of development, and comes in contact for a space with 

 the interior of the chorion. In ruminants it is very soon elongated 

 into two attenuated tubes which extend towards the ends of the ovum. 

 In carnivora it is of considerable size, stretching through the ovum 

 towards its opposite poles. 



In the human species it retains its vascularity and continues to 

 grow up to the fifth or sixth week, at which time it has assumed a 

 pyriform shape, and is connected by a tubular vitelline duct with the 

 intestine. 



But notwithstanding all these varieties of form and development of 

 the yolk-sac in the more advanced stages, we recognise the same 

 fundamental structure and relations to other parts as in oviparous 

 animals. Thus in human embryoes of from two up to four weeks there 

 have been observed the same progressive changes from the wide com- 

 munication of the yolk-sac with the open primitive intestine, to the 

 narrower vitello-intestinal aperture, and the subsequent elongation of 

 this into a tubular vitello-intestinal duct (figs. 511 and 51a.). 



The human yolk-sac or umbilical vesicle, which expands i)roportion- 

 ally with the early increase of the ovum, never, however, surpasses the 

 size of a small pea ; in an ovum of from five to six weeks it lies loosely 

 in the space between the amnion and chorion. At a later period, the 

 duct elongating with the umbilical cord, the vesicle remains in the 

 same relation to these membranes : it now also becomes flattened and 

 more closely attached in the narrower space remaining between 

 them. In the third month it is found connected with a coil of intestine 

 which in the form of a hernia occupies the umbilical cord outside the 

 abdomen of the embryo. At a later period the much elongated and 

 attenuated duct with the omphalo-mesenteric vessels, now impervious 

 and shrunk, may still be traced through the umbilical cord, while the 

 flattened vesicle may be found, even up to the end of the term of 

 nterogestation, somewhat further removed from the place of implanta- 

 tion of the umbilical cord on the internal surface of the placenta, but 

 still between the amnion and chorion. 



The Amnion. — This vesicular covering of the embryo does not exist 

 in amphibia and fishes, but in reptiles, birds, and mammals, it is formed 

 at an early stage of development, and contains a fluid in which the 

 foetus is suspended by the attachment of its umbilical cord or an 

 equivalent uniting medium. 



The structure of the amnion is essentially similar in the three classes 

 of animals in which it exists and its mode of formation nearly the same. 

 It is destitute of blood-vessels, and consists of two layers, derived 

 respect ivel)^, the inner from the epiblast, and the outer from the somato- 

 pleure layer of the mesoblast ; the first consisting of distinct nucleated 

 cells, the second presenting a fibrous structure. To its external or 

 fibrous layer it also owes the property of muscular contractility, which 

 it possesses in a considerable degree. 



The formation of the amnion takes place by the gradual backward 

 inflection from the flat part of the blastoderm adjoining the embryo of 

 the two layers before mentioned, first at the cephalic, and a little later at 

 the caudal extremity and at the sides (see fig. 512, 2, 3, and 4, Jcs, ss, am), 

 so that the two layers of which the amnion is composed are lifted up and 

 separated from the remaining two lower layers (splanchnopleure" and 

 hypoblast) of the blastoderm, by a space which is the same as, or rather 



