MAMMALIAN EMBRYOLOGY 625 



icle actually burrows its way into the mucous membrane lining of the 

 uterus. 



It is this third type which occurs in man and in some of the rodents 

 such as the guinea pig and the gopher. 



The trophoblast, in the region where it is to meet with the uterine 

 wall, has become highly specialized physiologically in the eccentric and 

 interstitial types of implantation. Its cells form a layer of considerable 

 thickness and it is then called a trophcderm (Fig. 358). These cells are 

 supposed to dissolve or digest the uterine mucosa so as to permit a defi- 

 nite implantation and also, probably, to digest some of the mucosa as 

 food for the growing embryo. 



The blastoderm attaches itself to the uterine wall between the two 

 oviducts, and it is in the region of implantation that the maternal tissues 

 come into contact with the embryo. We must, therefore, look for the 

 beginnings of the placenta in this region. 





Fig. 358. 



A. Diagrammatic section of placenta. (After Strahl, Bonnet.) i 



B. Section through an embryo of 1 mm. embedded in the uterine mucosa 

 (semidiagrammatic after Peters). Am:, amniotic cavity; b.c., blood-clot; b.s., 

 body-stalk ; cct., embryonic ectoderm ; ent., entoderm ; mes., mesoderm ; m.v., 

 maternal vessels ; tr., trophoderm ; u. e., uterine epithelium ; u.g., uterine glands ; 

 y.s., yolk-sac. 



In fact, it is the trophoderm which later becomes vascularized from 

 the mesoderm of the chorion or allantois, to act as the chief absorptive 

 surface through which, and by which, material from the maternal tissues 

 and blood is taken to the embryo. 



THE EMBRYONIC MEMBRANES 



It will be remembered that in the chick embryo, the amnion has as 

 one of its functions the protection of the embryo from drying and from 

 becoming deformed by the outer shell pressing against it. The chick's 

 yolk-sac contains a large quantity of food-substance which the develop- 

 ing embryo uses, and the allantois serves as a respiratory and (partially) 

 as an excretory organ. In the chick the serosa or chorion was of little 

 importance. 



