MAMMALIAN EMBRYOLOGY 





body-stalk extends, the allantoic stalk extends further along in the body 

 stalk as well, and so remains during foetal life (Fig. 360). 



As the ventral body-walls of the embryo are formed and approacl 

 each other, the proximal end of the allantoic stalk becomes the urinar 

 bladder and the beginning of the urogenital sinus. From the bladde 

 region to the body-wall it is reduced as a mere solid strand of connectiv 

 tissue known as the urachus. 



Vascularization is quite alike in the various mammalian forms. 



The development of the placenta depends upon the manner an< 

 type of implantation, which in turn causes different relationships betweei 

 the growing embryo and the maternal tissues. 



THE DECIDUAL MEMBRANES 



We have been describing the embryonic placenta. Now we shal 

 describe the maternal placenta. There is a change which takes place ii 

 the lining of the uterine walls when the trophoderm unites with thi 

 uterus. The uterine lining which bulges out into the uterine cavity t< 

 cover the blastoderm is called the decidua capsularis (formerly, decidu; 



reflexa), while the uterine lining a 

 the point where blastoderm am 

 uterus unite is called the deciduj 

 basalis or decidua serotina, the re 

 maining portion of the lining being 

 known as the decidua vera (Fig 

 361). 



The chorion is at first composec 

 of an inner mesodermal layer anc 

 an outer epithelial layer (this lattei 

 being called the trophectoderm) 

 From the trophectoderm there de 

 velops an outer syncytial layei 

 which is called the trophoderm. It 

 is this trophoderm which invades 

 the maternal tissues. Large lacunae of blood are formed in the mater- 

 nal tissues by the syncytial tissue directly, or by the rupture of the blood 

 vessels which are under great pressure in this region. 



The trophoderm then thickens at intervals and forms little villi or 

 finger-like projections, and the chorionic mesoderm grows out into these 

 villi so that there is a branching of the primary villi into secondary villi 

 or true villi (Fig. 358)). 



In the meantime the blood lacunae run together and surround and 

 bathe the villi, while the trophoderm, which began as a spongy network, 

 is now a continuous layer covering the entire outer surfaces of the villi 

 and chorion. 



Branches of the umbilical vessels develop in the mesoderm of the 





Fig. 360. 



Medial section of early human embryo. 

 ( After von Spec, Kollmann. ) 



