CHAPTEE X. 



DEVELOPMENT OF THE IMPREGNATED EGG AND 

 ITS MEMBRANES IN THE HUMAN SPECIES. AM- 

 NION AND CHORION. 



IN the human species, as well as in the lower animals, the foetus is 

 enveloped in two membranes, an inner and an outer, derived respec- 

 tively from extensions of the external and internal blastodermic layers, 

 and consequently parts of the emb^onic organism. While the inner 

 of these envelopes has the same characters as elsewhere, the outer one 

 presents such modifications of structure as to have received a distinct 

 name. In the lower 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 

 same manner as that already described (p. 740), namely, by the growth 

 of a circumvallation or fold of the external blastodermic layer, which 

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

 back of the embryo, thus inclosing it in a distinct cavity. 



Fig. 259. 



Fig. 260. 



HUMAN EMBRYO AND ITS ENVEL- 

 OPES, about the end of the first month. 

 1. Umbilical vesicle. 2. Amnion. 3, 

 Chorion. 



HUMAN EMBRYO AND ITS ENVEL- 

 OPES, at the end of third month; showing 

 the enlargement of the amnion.. 



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

 of the embryo, so that there is hardly any space between the two ; the 

 opposite surfaces lying in contact with each other, like those of the 

 48 ( 745 ) 



