Embryology. 19 



the yelk duct which (latterly) passes through the skin stratum on the 

 belly side of the embryo. 



The yelk sac is early furnished with bloodvessels by which its nutri- 

 ment is taken up and carried into the heart of the embryo. As the de- 

 velopment of the embryo advances, this nutriment in the yelk sac is 

 gradually consumed, and another apparatus is substituted in its place 

 for supplying the necessary nutrition. This is the allantois or sausage. 

 It is to be noted that the original use of the allantois was not to nourish, 

 and in the case of the higher mammals it is simply adapted or modified 

 to suit that end. 



The allantois is at first a little pouch or bladder which is developed 

 near the rear end of the intestinal tube. It is formed as a continuation 

 or rather branch or diver ticulum of that tube and is composed of the 

 same layers that form the tube and yelk sac. The first office of the al- 

 lantois is that of a urinary bladder, and it receives the urine from the 

 growing embryo. It protrudes from the intestinal tube through the 

 wall of the body cavity much as the yelk sac does, and as the yelk sac 

 decreases in size the allantois gains. It is early supplied with blood 

 vessels. As it grows it pushes its way until its end comes into contact 

 with the wall of the chorion. See Fig. 22 No. 5, cl. 



The chorion has by this time connected itself by means of its villi or 

 tufts with the inner wall of the uterus. When the membrane of the al- 

 lantois reaches the chorion, the blood vessels of the allantois penetrate 

 through into the hollow tufts on the outside of the chorion. This 

 brings the blood of the embryo into close proximity to the blood of the 

 mother, and enables the latter to supply its elaborated nutriment to the 

 former. The two bloods do not, however, flow directly into each other, 

 but that of the embryo absorbs nutrition from the mother's blood through 

 the walls of the villi Or tufts. 



The junction of the allantois to the chorion covers a flat area circular 

 in the human being, and when the embryo is mature it is a thick cake 

 six or eight inches in diameter. It is called the placenta. The stem 

 of the allantois then shrivels up into a cord a half inch in diameter, and 

 which is finally eighteen or twenty inches long, which is then called the 

 umbilicus. When the yelk sac is exhausted of its nourishment its re- 

 mains are pinched off from the intestinal tube. After which the belly 

 wall closes over and no trace of the yelk duct is left, either in the outer 

 wall of the body cavity or on the intestinal tube. The point, however, 

 where the umbilicus passes through the wall of the body cavity, leaves 

 its permanent mark in the "navel," because the wall closes up around 

 the cord while it is still in active use. 



Now return to the development of other parts which has been going 

 forward at the same time. On the front end of the medullary tube, a 



