THE AMNION. 599 



increase in its dimensions, the amnion forms a sheath around 

 the umbilical cord, and also comes into close contact with the 

 wall of the blastodermic vesicle over the whole extent of its inner 

 surface. 



The lic[uor amnii, which occupies the amnionic cavity, between 

 the amnion and the embryo, varies much in quantity at different 

 periods of gestation. It is apparently most abundant about the 

 fifth or sixth month. Its actual quantity is difiBcult to fix, as it 

 varies greatly in different cases ; when in excess, i.e. more than 

 about 1^ litre, it constitutes the affection known as hydrops 

 amnii. 



The liquor amnii contains urea, especially during the later 

 months of gestation ; this appears to be a true excretory pro- 

 duct, separated by the kidneys of the foetus, and discharged 

 through the urino-genital aperture into the amnionic cavity. 



Structurally, the human amnion consists, like that of the 

 rabbit or chick, of a single layer of epiblast cells, supported on a 

 thin layer of mesoblast (cf. Figs. 182 to 184). The mesoblast 

 consists of a homogeneous matrix, with embedded cells ; while 

 the epiblastic epithelium is, according to Minot, noteworthy on 

 account of the distinctness with which the intercellular bridges 

 of protoplasm, connecting the several cells with one another, can 

 be made out; the boundaries between adjacent cells being 

 formed, not by divisional planes, but by lines of vacuoles, 

 between which the protoplasmic bodies of the cells are directly 

 continuous with one another. 



2. The Umbilical Cord. 



The umbilical cord, which connects the embryo with the 

 placenta (Fig. 254), is formed in the first instance by the 

 allantoic stalk (Fig. 179, 197, tz). This stalk, in the human 

 •embryo, is directly continuous, from the first, with both the 

 embryo and the wall of the blastodermic vesicle (Figs. 186 to 

 188), and has the appearance of a direct prolongation back- 

 wards of the hinder end of the embryo. On the formation of 

 the tail, the allantoic stalk is gradually driven down to the 

 ventral surface of the embryo (cf. Figs. 179, 197,. 198, tl and 

 Tz), and acquires the position and relations characteristic of 

 it in rabbit or chick embryos at corresponding periods. 



The chief purpose of the allantoic stalk is to afford a path, 



