122 



THE HUMAN EMBRYO. 



on the opposite side it has developed numerous outgrowths, most of which are 

 formed exclusively of the ectoderm, but a few contain an ingrowth of mesoderm 

 in their interior. The ectoderm on the side toward the uterus has two layers, an 

 inner cellular layer with relatively small nuclei, and an outer syncytial or tropho- 

 blastic layer with larger nuclei of variable size. The ovum occupies a depression 

 on the surface of the uterus from which the uterine tissues have disappeared, 

 with the result of breaking through the walls of some of the blood-vessels, bl.lac, 

 so that now the maternal blood may escape from these vessels into the spaces left 

 between the irregular outgrowths and the embryonic chorion. We must assume 



Tro. 



Am.c. 



EC. 



Gl. bl.lac. Conn. Gi. 



FIG. 57. BLASTODERMIC VESICLE OF A MONKEY (SEMNOPITHECUS NASICUS) ATTACHED TO THK UTERUS ; 



VERTICAL SECTION. 



Ain.c, Amniotic cavity, bl.lac, Blood lacuna. Coe, Extra-embryonic coelom. Conn, Connective tissue of the 

 uterus. EC, Ectoderm. Gl, Gl, Uterine glands. Mes, Mesoderm of embryonic chorion. Sh, Embryonic 

 shield. Tro, Trophoblast. Vi, Mesodermic core of a chorionic villus. Yk, Yolk-sac. (After . Selenka.) 



that the trophoblast of the embryo has actually dissolved away or digested the 

 tissues of the uterus, thus providing an attachment for the ovum, securing its 

 embedding in the wall of the uterus, and establishing an opportunity for the 

 maternal blood to flow into the intervillous spaces. In later stages of the pri- 

 mates the trophoblast is very much reduced, and therefore fulfils its functions 

 in the^ery earliest stages by establishing these primitive conditions of blood- 

 supply. 



