HUMAN EMBRYO IN THE SECOND STAGE. 



129 



irregular outline and contained many large spaces filled with maternal blood (Fig. 

 74). The exact external diameter of the ovum could not, therefore, be determined. 

 It measured, however, approximately 2.4 mm. by 1.2 mm. The internal diameter 

 of the chorionic vesicle was about 1.6 by o . 8 mm. The trophoderm is every- 

 where intimately united with the uterine tissue. The embryo, Sh, is represented 

 by an embryonic shield consisting of cylinder cells. It is small and lies on the 

 side of the ovum away from the cavity of the uterus. It rests upon the small 

 yolk-sac, Yk, and is overlain by the amniotic cavity, Am. c, which is bounded every- 

 where by ectoderm on one side, of course, that of the embryonic shield; on the 



A.mes. 



Cce. 



Ent. 



FIG. 73. EMBRYO OF THE PRECEDING FIGURE MORE HIGHLY MAGNIFIED. 



Am.c, Amniotic cavity. A.ec, Amniotic ectoderm. A.mes, Amniotic mesoderm. b.s, Body-stalk. Cce, Extra- 

 embryonic ccelom. Ent, Entoderm. mes', Somatic, mes", splanchnic, mesoderm. Sh, Embryonic shield. 

 (After E. Selenka.) 



other the thin amniotic ectoderm proper. The mesoderm extends around the ovum, 

 forming a layer underneath the chorionic ectoderm over the yolk-sac and above the 

 amnion. At one point, close to the embryo and yolk-sac, it encloses a triangular 

 space the meaning of which is not known. As indicated in the figure, the meso- 

 derm was found to have shrunken somewhat, and the appearance of the embryo 

 and yolk-sac also suggests a somewhat imperfect preservation, histologically speak- 

 ing, of the tissues. As regards the condition of the uterus, the following points may 

 be noted. In the neighborhood of the ovum the decidua vera had acquired a 

 thickness of about 8 mm., while on the opposite or anterior side it was only from 

 5 to 6 mm. in diameter. Only in the immediate neighborhood of the ovum could 

 there be seen any differentiation of the mucous membrane into an upper, more 

 compact layer, and a deeper, looser cavernous layer. The epithelium of the glands 

 and the tissues of the uterus were well preserved, except in the immediate neighbor- 

 hood of the ovum. The picture produces the impression that the ovum, in order 

 to secure a place for itself, has completely destroyed the uterine tissues with 

 which it has been in contact, thus implanting itself in the maternal tissue. As a 



9 





