124 THE HUMAN EMBRYO. 



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 includes as 

 yet very little, if any, mesoderm. Probably a neurenteric canal exists at this stage. 

 During the transition of stage B (Fig. 68) to stage C (Fig. 69), the blood-vessels 

 appear in the mesoderm of the yolk-sac. 



Relations of the Embryo to the Uterus: the Two Stages. 



The Two Stages. During the first half or perhaps five months of pregnancy 

 the decidua reflexa is present. This period is called the first stage, to distinguish 

 it from the remaining period, or second stage, during which there is no decidua 

 reflexa. The reflexa during the first stage grows very thin and at the same time 

 degenerates. It is finally resorbed. The exact date of its disappearance is not 

 known, but falls somewhere in the fifth month. During the first stage the chorion 

 laeve is in contact with the decidua reflexa, during the second stage with the 

 decidua vera. On pages 343 and 345 a typical uterus of each stage is described. 



The First Stage. The study of young human ova 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 researches. The essential idea we have formed of implantation is 

 that the trophoderm 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 portion 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, first, modifications in the glands, the epithelium of which in large 

 part degenerates; second, 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, third, by a growth of its blood- 

 vessels. 



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

 decidua serotina, the area (Fig. 70, s,s) to which the ovum is attached; second, the 

 decidua vera, comprising all the remaining portions of the mucosa forming part of 

 the walls of the body of the uterus; third, the decidua reflexa, the arching dome of 



