118 THE HUMAN EMBRYO. 



The amnion, Am, is now completely separated from the chorion, Cho, which forms 

 a relatively large vesicle and consists of a thin layer of mesoderm, and a very 

 thick layer of ectoderm, which has an inner cellular stratum and an outer very 

 much thicker trophoblastic stratum. The trophoblast is now very much altered 

 by the appearance of numerous spaces or channels in it which develop so that 

 each of these spaces ends blindly toward the interior of the chorion, but many of 

 them are open upon the surface of the trophoblast. As the ovum at this stage is 

 already embedded in the uterine mucosa, the channels in the trophoblast can 

 receive maternal blood, and such is their original function. The embryo and 

 yolk-sac, as compared with the chorionic vesicle, are very small in size. The 

 body-stalk, b. s., is well developed and contains a well-marked allantoic anlage, 

 All, formed by the entoderm. The embryo contains as yet very little, if any, 

 mesoderm. Probably no neurenteric canal exists at this stage. During the 

 transition of stage B (Fig. 53) to stage C (Fig. 54), the blood-vessels appear in 

 the mesoderm of the yolk-sac. 



Relations of the Embryo to the Uterus. 



The study of Peters's ovum and of early stages of various primates leads us 

 to conceive that the ovum first implants itself in the mucous membrane of the 

 uterus. The conception, "implantation," is the outcome of very recent re- 

 searches. The essential idea we have formed of implantation is that the tropho- 

 blast of the ovum corrodes or digests the uterine tissues with which it comes in 

 contact, and thus produces a cavity in which it is lodged and where it attaches 

 itself intimately to the maternal tissues. Owing to this process the ovum is at 

 first partly uncovered, and this condition seems to be permanent in monkeys. 

 In man and the apes, however, the uterine mucosa grows over the exposed por- 

 tion of the ovum, forming a layer of maternal tissue which separates the ovum 

 from the cavity of the uterus. This layer is the anlage of the decidua reflexa. 

 As the ovum grows, the decidua reflexa must also expand, and we soon reach a 

 condition in which the primitive relations of the parts can be easily followed. 



When the uterus becomes pregnant, the mucous membrane of the organ 

 undergoes changes in structure, and it is then commonly no longer termed the 

 mucosa, but the decidua or caduca. The decidual membrane is histologically 

 characterized by modifications in the glands, the epithelium of which in large 

 part degenerates, by the transformation of a large number of the connective- 

 tissue cells into cells of large size, which, on account of their being so extremely 

 characteristic, are called the decidual cells, and, finally, it is characterized by a 

 growth of its blood-vessels. 



The decidual membrane of the uterus is divided into three regions : first, the 



