602 THE HUMAN EMBRYO. 



2. The right and left allantoic arteries, u. These are usually 

 quite distinct from each other along the greater part of the 

 length of the cord, but just before reaching the placenta are 

 almost invariably united by an anastomotic branch. 



3. The allantoic vein. This has thinner walls than the 

 arteries, and has also, according to Kolliker, rudimentary valves. 

 There are at first two allantoic veins, but the right one is, almost 

 from the first, smaller than the left, and disappears completely 

 about the fourth week. 



4. The epithelial lining of the allantoic cavity. During the 

 first and early part of the second month, the allantoic stalk is 

 hollow, its cavity extending from the cloaca of the foetus along 

 the whole length of the cord, as far as the wall of the uterus. 

 Later on, in the third or fourth month, the cavity becomes con- 

 stricted, or altogether obliterated. Isolated portions of it may, 

 however, persist, especially at the proximal or foetal end of the 

 cord, up to the time of birth. 



5. The yolk-stalk and its vessels, the vitelline arteries and 

 veins. These usually disappear during development, and are 

 seldom to be distinguished in the cord at full time. The yolk- 

 stalk at first lies in a groove in the allantoic stalk ; but it soon 

 becomes completely surrounded by this latter, and then ceases to 

 be distinguishable. 



G. The Whartonian jelly: this forms the matrix of the cord, 

 in which are embedded the various structures described above. 

 It consists of a complex network of branching connective-tissue 

 cells, embedded in a clear gelatinous matrix. Immediately 

 beneath the surface epithelium, and around the blood-vessels 

 and the allantoic cavity, the connective-tissue meshwork is rather 

 denser than elsewhere ; and, in the matrix, fibres are developed, 

 especially during the later months of gestation. 



7. Up to the close of the third month, the end of the umbilical 

 cord next the embryo contains, as already noticed, a loop of the 

 intestine (Fig. 254, i), but after this date the alimentary canal is, 

 as a rule, completely withdrawn into the body of the foetus. 



3. The Choi-ion. 



The term chorion has been used in very different senses by 

 writers on embryology. It is convenient to employ it for that part 

 of the blastoderm, or blastodermic vesicle, which is not directlj 



