1 90 EMBRYOLOGY. 



from the mother, and the remains of the yelk-sac (umbilical ves- 

 icle) dwindle as the need is less and the substance is consumed. 

 (See Figs. 60, 61, and 62.) 



What structures in the body are derived from the beginning 



of the allantois ? 



The urinary bladder and the urachus, an impervious cord extend- 

 ing from the bladder to the umbilicus. 



How is the chorion made up ? 



It consists of several layers which fuse into one vascular mem- 

 brane. The allantois, the outer layer of the amnion, and the vitel- 

 line membrane are united in the chorion. As the embryo develops 

 the vessels of the chorion become thinner on the side toward the 

 uterine cavity, and more distinct on the opposite side. This change 

 continues as the embryo increases, until the placenta is formed by 

 the branching of the embryonic vessels and the increase of the 

 decidua at the corresponding point. 



What changes occur in the uterus in developing the placenta ? 

 In the deeper part of the mucous membrane of the uterus at the 

 implantation of the chorion there are hollowed out spaces or si- 

 nuses in the tissues which communicate both with a maternal vein 

 and an artery ; that is, special arrangements are made for the rapid 

 circulation of a large amount of blood in the uterine mucous mem- 

 brane at the placental site. At the same time, the glandular struc- 

 tures of the uterine mucous membrane are increased, and the fol- 

 licles run deeply into the thick and succulent tissues. 



How do the villi of the chorion become implanted in the uterine 



wall? 



The villi dip down and develop new tufts of capillaries in the 

 deepened crypts of mucous membrane, so that the tufts of capil- 

 laries of the chorion may be said to resemble in a way a glove filled 

 with foetal blood dipping into a vessel filled with maternal blood. 



Does the blood of mother and foetus actually come in contact ? 

 No. There are four layers of cells between the maternal and 

 the foetal blood : 1, wall of chorion capillary ; 2, cells of chorion ; 

 3, cells of uterine follicle ; and 4, wall of the uterine sinus. 



Do the tissues of the placenta maintain an exchange of mate- 

 rial? 

 Yes. The mother's blood furnishes to the fo3tal blood food and 



