90i 



EMBBYOLOar. 



COMPARISON OF THE ANNEXES OP THE HOMAN ECETUS -WITH THOSE OP ANIMALS. 



The human fcetus, like that of the domesticated mammals, is enveloped by an amnion 

 and chorion which are generally identical in disposition with those already described. 

 The umbilical vesicle submits to the same change as in the Mare, becoming so quickly 

 atroohied that scarcely any traces of it can be found at birth. It is impossible to 

 isolate the amnion from the inner face of the chorion; so that some anatomists only 

 admit its presence by analogy with what is observed in animals. 



Fig. 431. 



^""IkhilMI 



DIAGRAM OP AN EAELY HUMAN OVUM, 

 SHOWING THE AMNION IN PKOOESS OP 

 POEMATION, AND THE ALLANTOIS BEGIN- 

 NING TO APPEAR. 



a, Chorion ; 6, Vitelline mass, surrounded by 

 the blastodermic vesicle ; c, Embryo ; d, e,f, 

 External and internal folds of the serous 

 layer, forming the amnion; g, Incipient' 

 allantois. 



DIAGRAM OP A HUMAN OVUM IN SECOND 

 MONTH, SHOWING THE COMPLETION OP 

 THE SAC OP THE AMNION, AND A PUR- 

 THER DEVELOPMENT OP THE ALLANTOIS. 



al, Smooth portion of the chorion; a2, Its 

 villous portion ; k, k, Elongated villi col- 

 lecting into placenta ; 6, Vitelline, or um- 

 bilical vesicle ; c. Embryo ; /, Amnion, inner 

 layer; h, Its outer layer coalescing with 

 the chorion. 



The umhilical card offers nothing particular. The placenta is circular; its diameter 

 at the termination of pregnancy being from 6 to 8 inches, and its thickness from 1 to IJ 

 inclies. There is distinguished the foetal placenta, m the midst of which the umbilical 

 coril arrives ; and the maternal placenta, whose villosities dovetail witli those of the 

 former. 



The insertion of the placenta takes place towards the fundus of the uterus, near one 

 Of the Fallopian tubes. When there are more than one fcetus, there are a corresponding 

 number of placentae. 



Independently of those annexes, which are the same in all, there is described for the 

 human fcetus a special envelope, external to the chorion ; this ia the membrana decidua. 

 This is formed by the hypertrophied mucous membrane, which is doubled around the 

 ovum, when the latter is lodged in the uterine cavity. Consequently, there results the 

 division of the decidua into two parts : (he true decidua (decidua vera) which covers the 

 uterus, and tbo reflected decidua (decidua reflexa) which envelops the chorion. These 

 present the characters of the hypertrophied uterine mucous membrane. 



