THE CHORION THE DECIDTJA. 93 



uro- genital sinus, and it is to the urinary tract that the allantois mainly belongs. 

 It projects out from the embryo through the same opening as the vitelline or 

 umbilical duct. The lower part of the allantois which is contained within the 

 embryo becomes the bladder ; the upper part of its intra- embryonic portion is 

 denominated the urachus; the extra-embryonic portion is divided into two 

 parts, called the allantoic vesicle, or the epithelial portion of the allantois, and 

 the fibrous or vascular portion. The allantois, which is at first a simple serous 

 membrane, becomes vascular over its whole extent about the fifth week, and 

 its vessels communicate, as will be described presently, with those of the cho- 

 rion, forming the vascular connection between the mother and foetus. In the 

 human foetus the allantoic vesicle is small, soon withers and disappears, and its 

 vessels are soon limited to the two umbilical arteries and one vein. 



The allantoic fluid is alkaline, and contains from one to four per cent, of 

 solid matters uric acid, urea, allantoin, sugar, and saline matters. 



Umbilical Vesicle (Figs. 53, 54, o). The embryo itself in the earliest recog- 

 nizable condition is, above stated, a mere streak, but it soon becomes curved at 

 either end, corresponding to the head and lower extremity of the future animal 

 (the tail of animals, the buttocks and lower limbs in man); the lower part is, 

 however, open, and from this a body projects which at first consists of the 

 matter into which the yelk has been developed (yelk-sac), and later on is con- 

 verted into a vesicular body filled with clear fluid (the umbilical vesicle), and 

 communicating with the body of the embryo by a constriction, the umbilical 

 duct, opening at first into the intestinal cavity. As the development of the 

 intestine proceeds, this canal is closed, and the umbilical vesicle is then a closed 

 sac, lying external to the arnnion. It is formed mainly by the internal germi- 

 nal layer, but has a lining derived from the middle layer. As the arteries 

 developed in the middle layer grow they cover the umbilical vesicle, forming 

 the vascular area, the chief vessels of which are the omphalo-mesenteric, two in 

 number. The vessels of this area appear to absorb the fluid of the umbilical 

 vesicle, which dries up into a disk-like body attached to the amnion, and hav- 

 ing no further function. The activity of the umbilical vesicle ceases about the 

 same time (fifth or sixth week) as the allantois is formed. In fact, the umbilical 

 vesicle provides nutrition to the foetus from the ovum itself, while the allantois 

 is the channel whereby nutrition is conveyed to it from the uterine tissues. 

 The umbilical vesicle, however, is visible, containing fluid up to the fourth or 

 fifth month, between the amnion and the chorion, with its pedicle and the 

 omphalo-mesenteric vessels. The latter vessels then become atrophied, as the 

 functional activity of the body with which they are connected ceases. 



The Chorion (Figs. 55, k, and 56). The primitive chorion has already been 

 described. It is formed by the vitelline membrane, which becomes covered 

 with shaggy villous processes, and disappears about the fifteenth day, to give 

 place to the secondary or permanent chorion. The latter is composed of two 

 lamella, the external one of which is furnished by the vesicula serosa (or 

 false amnion), and the internal by the fibrous layer of the allantois. This 

 latter furnishes a vascular membrane, which is applied to the epithelial layer 

 of the chorion (vesicula serosa) (Fig. 54, 13, 14, 15). As the latter becomes 

 villous by the development of tufts upon it (shaggy chorion), the bloodvessels of 

 the internal layer pass into those tufts, forming the foetal portion of the placenta, 

 and dipping through the decidua into the uterine sinuses of the maternal 

 placenta. 



The Decidua (Figs. 54, 55) is formed from the mucous membrane of the 

 uterus. Even before the arrival of the fecundated ovum in the uterus, the 

 mucous membrane of the latter becomes vascular and tumid, and when the ovum 

 has reached the uterus, it is imbedded in the folds of the mucous membrane, 

 which overlap, and finally completely encircle the ovum. Thus two portions 

 of the uterine mucous membrane (decidua) are formed viz., that which coats 



