CHAPTER IX. 



MEMBRANES OF THE IMPREGNATED EGG IN THE 

 HUMAN SPECIES. AMNION AND CHORION. 



IX man, as well as in many animals, the foetus is enveloped in two 

 membranes, an inner and an outer, derived respectively from the 

 external and internal blastodermic layers, and consequently parts of 

 the embryonic organism. While the inner of these envelopes has the 

 same characters in man as elsewhere, the outer presents such modifica- 

 tions of structure as to have received a distinct name. In animals, 

 therefore, the foetal membranes are called the amnion and the allantois ; 

 in man, they are known as the amnion and the chorion. 



Amnion. 



The formation of the amnion in the human species takes place in the 

 manner already described (page 641), namely, by the growth of a cir- 



FIG. 198. 



FIG. 199. 



HUMAN* EMBRYO AND ITS ENVELOPES, about 

 the end of the first month. 1. Umbilical 

 vesicle. 2. Amnion. 3. Chorion. 



HUMAN EMBRYO AND ITS ENVELOPES, at the end 

 of the third month ; showing the enlargement of 

 the amnion. 



cumvallation or surrounding fold of the external blastodermic layer, 

 which extends itself in such a way that its edges meet and coalesce 

 over the dorsal region, inclosing the embryo in a distinct cavity. 



At the time of its formation, the amnion closely embraces the body 

 of the embryo ; the opposite surfaces lying nearly in contact, with 

 hardly any space between the two. This space afterward enlarges and 



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