56 HUMAN EMBKYOLOGY. 



appears to exist in the human subject till the embryo is three weeks old and 

 about 2 '5 mm. long. During the fourth week the vitello-intestinal duct is 

 elongated into a relatively long narrow tube, which is lodged in the umbilical cord 

 and the yolk-sac, which has become a relatively small vesicle, is placed between 

 the outer surface of the amnion and the inner surface of the chorion, in the region 

 of the placenta (Fig. 62). During the latter part of the fourth or the early part of 

 the fifth week, when the embryo has attained a length of about 5 mm., the vitello- 

 intestinal duct separates from the intestine and commences to undergo atrophy, 

 but remnants of it may be found in the umbilical cord up to the third month. 



The yolk-sac itself persists until birth, when it is, relatively, a very minute 

 object which lies either between the amnion and the placenta or between the 

 amnion and the chorion laeve. 



At a very early period, before the paraxial mesoderm has commenced to divide 

 into mesodermal somites, a number of arteries, the primitive vitelline arteries, are 

 distributed to the yolk-sac from the primitive arterial trunks of the embryo, the 

 primitive aortse, and the blood is returned from the yolk-sac 'to the embryo by a 

 pair of vitelline veins (Fig. 81). 



After a time the arteries are reduced to a single pair, and after the two primi- 

 tive dorsal aortse have fused into a single trunk, the pair of vitelline arteries 

 also becomes converted into a single trunk, which passes through the umbilical 

 orifice along the vitello-intestinal duct to the yolk-sac (Fig. 83). 



The vitelline veins also pass through the umbilical orifice on their way to the 

 heart of the embryo, and they become connected together, in the interior of the 

 body of the embryo, by transverse anastomoses, which are described in the account 

 of the development of the vascular system. 



After the umbilical cord is formed, the extra-embryonic parts of the vitelline 

 veins disappear, and can no longer be traced in the cord. The same fate overtakes 

 the extra-embryonic and a portion of the intra-embryonic part of the vitelline 

 artery, and the remainder of the artery persists as the superior mesenteric. 



THE PLACENTA. 



The placenta is an organ developed for the purpose of providing first the 

 embryo and later the foetus with food and oxygen, and for removing the effete 

 products produced by the metabolic processes which take place in the growing 

 organism. It is formed partly from the zygote and partly from the mucous 

 membrane of the uterus of the mother. 



In the placenta the blood-vessels of the embryo of the earlier stages and 

 the foetus of the later stages and the blood of the mother are brought into close 

 relationship with one another, so that free interchanges may readily take place 

 between the two blood streams; and the modifications and transformations of 

 the uterine mucous membrane and the chorion of the zygote, by which this 

 intimate relationship is attained, constitute the phenomena of the development of 

 the placenta. 



The details of the development of the human zygote for the first ten or twelve 

 days after the fertilisation of the ovum are not known, but the knowledge of what 

 happens in other mammals justifies the belief that during that time the zygote is 

 formed, in the ovarian, or the middle part of the uterine tube, by the union of a 

 spermatozoon with the mature ovum. During the first ten to fourteen days after 

 its formation it passes along the uterine tube, towards the uterus, whilst, at the same 

 time, it undergoes the divisions which convert it into a morula. 



The Formation of the Placenta. Before the zygote reaches the uterus the 

 mucous membrane which lines the cavity of that organ undergoes changes, in 

 preparation for its reception and retention, and when the changes are completed 

 the modified mucous membrane is known as the uterine decidua. 



The changes which take place are, for the most part, hypertrophic in character ; 

 the vascularity of the mucous membrane is increased, mainly by the dilatation of 

 its capillaries; the tubular glands of the membrane are elongated, they become 



