110 



THE EARLY DEVELOPMENT OE MAMMALS. 



umbilical cord the neck of the yolk-sac, Vi, becomes constricted and very much 

 lengthened out, forming the yolk or vitelline stalk, Vi. s. The yolk-stalk springs 

 within the embryo from the wall of the intestine, runs through the ccelom of the 

 umbilical cord, and makes its exit beyond the amnion, as shown in the figure. 

 The yolk-sac proper still occupies its original position between the amnion and 

 chorion. The student should note carefully that the umbilical cord is never 

 covered by the amnion, for it has unfortunately been often stated that it is so 

 covered. An idea of the relations can be gathered from cross-sections (Fig. 51 ). 

 The coelom, Cir, is a large cavity and contains the yolk-stalk, Y, with two 

 blood-vessels, but with its entodermal cavity entirely obliterated. Above the 

 body-cavity is the duct of the allantois, All, lined by entodermal epithelium, and 

 in its neighborhood are two arteries and a single vein. In yet earlier stages there 



Fir;. 51. — Sections of Two Human Umbilical Cords. 



A, From an embryo of 21 mm. ; B, from an embryo of sixty-four to sixty-nine days. All, Allantois. 



h. Umbilical artery. Ca, Ccelom. v, Umbilical vein. )', Yolk-stalk. 



are two veins. The outer surface of the section is bounded by ectoderm. The 

 further development of the cord depends upon the growtli of the connective 

 tissue and blood-vessels, the abortion first of the ccelom, later of the yolk-stalk, 

 and lastly of the allantoic duct. Remnants of the allantoic epithelium are, 

 however, often found in the umbilical cord even at birth. There occurs also a 

 further differentiation of the connective tissue and of the entoderm. 



The umbilical cord is characteristic of mammals. It varies greatly in length. 

 In the pig it is very short and in man it attains great length and size, becoming 

 at full term about 55 cm. in length, and 12 mm. in thickness. When fully de- 

 veloped, it has a whitish color and presents a twisted appearance somewhat like 

 a loop. Its surface is smooth and glistening. The attachment of the cord to the 



