904 



EMBRYOLOGY. 



and 



COMPARISON OF THE ANNEXES OF THE HDMAN FCETUS WITH THOSE OF ANIMALS. 



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

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



L'he umbilical vesicle submits to the same change as in the Mare, becoming so quickly 

 atrophied 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. 



DIAGRAM OF AN EARLY HUMAN OVUM, 

 SHOWING THE AMNION IN PROCESS OF 

 FORMATION, AND THE ALLANTOIS BEGIN- 

 NING TO APPEAR. 



a, Chorion ; 6, Vitelline mass, surrounded by 

 the blastodermic vesicle ; c, Embryo ; c?, e,/, 

 External and internal folds of the serous 

 layer, forming the amnion; </, Incipient 

 allantois. 



DIAGRAM OF A HUMAN OVUM IN SECOND 

 MONTH, SHOWING THE COMPLETION OF 

 THE SAC OF THE AMNION, AND A FUR- 

 THER DEVELOPMENT OF 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 umbilical cord 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 1| 

 inches. There is distinguished the foetal placenta, in the midst of which the umbilical 

 cord arrives ; and the maternal placenta, whose villosities dovetail with 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 foetus, there are a corresponding 

 number of placentae. 



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

 human foetus a special envelope, external to the chorion ; this is 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 : the true decidua (decidua vera) which covers the 

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

 present the characters of the hypertrophied uterine mucous membrane. 



