96 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



the uterus is reached a third or decidual coat is added by the 

 mucous membrane of the uterus (Fig. 71). 



The serotinal part of the decidua marks the place where the 

 placenta will be formed. The implantation of the ovum in the 

 decidua is in the posterior wall of the uterus in over 60°/ o 

 of cases ; hence the placenta is developed there. It sometimes 

 happens that implantation occurs near the internal os of the 

 uterus, and in such cases the placenta is developed over the os 

 (placenta praevia), a form liable to give rise to a sudden haemor- 

 rhage from the uterus during pregnancy. 



The Body-Stalk. — When the amnion and prechorion are 

 formed from the human ovum that part of the somatopleure 



chorion 



direction of growth 



neurenteric canal or blastopore 



head fold of 

 somatopleure 



caoity of amnion 

 body stalk 



cauity of allantois 

 epiblast 

 chorion 



chorionic villus. 



Fio. 75.— Showing the growth backwards of the Somatopleuric Head Fold in the 

 human ovum of 15 days to form the Amnion and Prechorion. The "body- 

 stalk " part of the Somatopleure keeps the embryo attached to the Prechorion. 

 (After Graf Spee.) 



lying behind the neural groove, and in which the primitive streak 

 appeared (see Figs. 68, 74 and 75), joins the embryo to the 

 chorion. His named it the body-stalk. Through it the allantois 

 — a hollow protrusion from the hind gut (Fig. 75) — grows out 

 to the pre-chorion Afterwards the body-stalk, with the con- 

 taind stalk of the allantois, becomes the umbilical cord. The 



