THE HUMAN EMBRYO IN THE SECOND STAGE. 125 



tissue. The embryo, Sh, was 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 am- 

 niotic cavity, Am. c, which is bounded everywhere by ectoderm on one side, of 

 course, that of the embryonic shield ; on the 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 mesoderm was found to have 

 shrunken somewhat, and the appearance of the embryo and yolk-sac also sug- 

 gests a somewhat imperfect preservation, histologically speaking, 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 diam- 

 eter. 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 neighborhood 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. And as a conse- 

 quence of the destruction of the maternal tissues the walls of some of the blood- 

 vessels have been broken through, and this has allowed the blood to escape from 

 those vessels into the lacunae of the trophoblast. 



The trophoblast. of the ovum offers a very complex picture, owing chiefly 

 to the changes which it is undergoing. The changes seem here due apparently 

 to hypertrophic degeneration. The layer of the chorionic ectoderm next to the 

 mesoderm retains more or less evidently a cellular character. The remaining 

 portions tend to form a syncytium in which the nuclei become enlarged and the 

 cell-boundaries obliterated, while the protoplasm of the cells also changes in 

 character and becomes more homogeneous in texture and much denser. The 

 syncytium disappears by resorption, and its disappearance causes the formation 

 of spaces in the trophoblast. Many different pictures occur in connection with 

 these processes, for in some places the nuclei tend to gather in groups, in others 

 they disappear, in some instances strands of degenerative material are left, while 

 nearby some of the trophoblast may retain its more primitive appearance and be 

 but slightly altered. Finally, it should be" noted that at various points the 

 chorionic mesoderm is growing out into the trophoblast. Each of these meso- 

 dermic outgrowths is to be interpreted as the anlage of the central portion of a 



