232 



EMBRYOLOGICAL TYPES 



their way into the trophoblast. In this way the placenta is 

 formed, and since it is related to the allantois, it is called an 

 " allantoic placenta." The placenta is an organ which places 

 the mother and embryo in physiological communication, for 

 the interchange of substances. The epithelium of the wall 

 of the uterus disappears where the trophoblast touches it, with 

 the result that the trophoblast is in contact with the subepithelial 

 tissues and blood-vessels of the uterine wall. The blood from 

 these maternal vessels bathes the surface of the trophoblast. 





Fig. hi 



-Section through a part of the allantoic placenta of the rabbit. 



The maternal tissue is on the right, the embryonic tissue on the left. 

 They can often be distinguished by the fact that the red blood-corpuscles 

 of the embryonic blood have not yet lost their nucleus, a, allantois ; ct, 

 cyto-trophoblast ; eb, embryonic blood-vessels ; eg, embryonic glycogenic 

 layer ; /, lacunas in the trophoblast and filled with maternal blood ; mb, 

 maternal blood-vessels ; mg, maternal glycogenic layer ; st, syncytium or 

 plasmodi-trophoblast ; uv, umbilical vein. 



Further, the trophoblast, which in this region is now thick, 

 becomes hollowed out by a number of spaces or lacunae ; and 

 these lacunae become filled by maternal blood which oozes 

 out from the wall of the uterus. 



The capillaries of the allantois branch in the substance of 

 the placenta, and the blood which they contain is separated 

 from the maternal blood only by the lining of the capillaries 

 and the surface of the trophoblast. (The blood of mother and 

 embryo are never in direct communication.) Across these, 

 substances are passed by diffusion. The maternal blood 

 supplies not only oxygen but food, and the embryonic blood 



