116 



.THE EARLY DEVELOPMENT OF MAMMALS. 



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

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

 development of the cord depends upon the growth of the connective tissue and 

 blood-vessels, the abortion first of the coelom, 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. In man it attains great length and size, becoming at 

 full term about 55 cm. in length and 12 mm. in thickness. When fully^ developed 

 the human cord has a whitish color and presents a twisted appearance somewhat 



FIG. 66. 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. Ar, Umbilical 

 artery. Cce, Coelom. v, Umbilical vein. Y, Yolk-stalk. 



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

 the embryo is known as the umbilicus. Its attachment to the chorion is in the pla- 

 cental region (chorion frondosum). 



The twisting of the cord is well marked externally at the time of birth by the 

 spiral ridges, within each of which a large blood-vessel runs. The number of spirals 

 varies from 3 to 32, the turns beginning at the embryo, and, though usually from 

 left to right, are sometimes from right to left. The twisting begins about the middle 

 of the second month. Its cause is unknown, but there is no reason to assume that 

 it is due to revolutions of the embryo. The cord is covered by a layer of epithe- 

 lium which is continuous at the distal end with the epithelium of the amnion, and 

 at the proximal end with the epidermis of the embryo. The cord contains typically 

 no capillaries, and, except in the immediate neighborhood of the embryo, no nerve- 

 fibers. 



