128 



THE HUMAN EMBRYO. 



fulfills its functions in the very earliest stages by establishing these primitive condi- 

 tions of blood-supply. 



A section of the embryo on a larger scale is shown in figure 73. There ap- 

 pears only the embryonic shield, Sh, which is remarkable for its small area and 

 great thickness. The yolk-sac is also very small and is lined by a distinct layer of 

 entoderm, Ent. Above the embryonic shield is the amniotic cavity, which is, of 

 course, bounded by ectoderm which is continuous with the ectoderm of the embry- 

 onic shield. The amniotic cavity has a curious extension into the body-stalk, b.s, 



Tro. 



Cce. Mes. Sh. Yk. 



Ant.c. 



EC. 



Gl. bl.lac. Conn. Gl. 



FIG. 72. BLASTODERMIC VESICLE OF A MONKEY (SEMNOPITHECUS NASICUS) ATTACHED TO THE UTERUS; VERTICAL 



SECTION. 



Am.c, Amniotic cavity, bl.lac, Blood-lacuna. Cce, 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 E. Selenka.) 



by which the embryo is connected with the chorion. The mesoderm is chiefly de- 

 veloped over the chorion, as shown in figure 72. It is very slightly developed in 

 the embryo (Fig. 73, mes), but forms a layer over the yolk-sac and over the am- 

 nion, and forms a considerable mass of tissue to constitute the body-stalk, b.s. 



Human Embryo in the Second Stage. 



The embryo to be described was investigated by H. Peters. It was found 

 attached to the dorsal wall of a uterus almost completely embedded in the mucosa, 

 but it was not wholly covered thereby, so that there was no decidua reflexa yet 

 present. A blood-clot overlay what would have been otherwise the exposed por- 

 tion of the ovum. The trophoderm formed an enormously thick layer of very 



