126 



THE HUMAN EMBRYO. 



more or less smooth. The inner surface of the reflexa is more irregular and 

 has protuberant parts united with the tips of the chorionic villi. The surface 

 of the decidua serotina, on the contrary, becomes very irregular during the 

 progress of pregnancy, forming little mounds which may become so high as to 

 resemble columns or so broad as to constitute septa. In later stages the septa 

 become very well developed, attaining a height of from 5 to 15 mm. They are 

 irregularly disposed, but subdivide the placenta of later stages into the so-called 

 cotyledons (compare page 362). 



Muse. 



OV. 



-Ovd. 





FIG. 71. HUMAN UTERUS, ABOUT FORTY DAYS ADVANCED IN PREGNANCY. 



Muse, Muscularis. Dv, Decidua vera. D.ref, Decidua reflexa. Ov, Ovary. Ovd, Oviduct (Fallopian tube). 

 Lig, Round ligament. Vg, Vagina. The uterus has been opened by cutting through the anterior walls 

 and reflecting the sides. (After Coste.) 



The Second Stage. The body-stalk becomes converted into the umbilical cord. 

 This cord runs from the body of the embryo to the chorion (Figs. 70 and 87). It 

 is always connected with that portion of the chorion which is adjacent to the de- 

 cidua serotina. It carries the arteries and veins from the body of the embryo to 

 the chorion. From the end of the umbilical cord the blood-vessels branch out over 

 the chorion and into the chorionic villi. Thus the chorionic circulation of the em- 

 bryo centers about the chorionic end of the umbilical cord, and, as this end is in the 

 part of the chorion overlying the decidua serotina, we have here established from 



