900 EMBRYOLOGY. 



villi The fusion of these two systems has never been observed, and all the 

 interchanges between the female and its young take place through the 

 capillaries by osmotic force only. 



6. The Umbilical Cord. 



The cord is formed by the vessels which, in the foetus, carry the 



blood to the envelopes, and chiefly to the 

 placenta. It is divided into two portions : 

 an amniotic, the longest, which is always 

 twisted on itself like a cord, and covered 

 externally by the amnion that is prolonged 

 on its surface, to be continued with the 

 skin around the umbilicus ; the other, the 

 allantoid portion (Fig. 425, b), much 

 shorter and less twisted, is enveloped by the 

 sheath that continues the two laminae of 

 the allantois, and is inserted into the supe- 

 rior wall of the chorion, between the two 

 cornua. 



Three vessels compose the cord: two 

 arteries and a vein ; these are covered by 

 a layer of embryonic tissue, the gelatine of 

 Wharton, which makes them appear much 

 larger than they really are. 



The Umbilical Akteeies arise from 

 the internal iliac, and pass along the sides 

 of the bladder ; escaping by the umbilicus, 

 they arrive at the terminal extremity of 

 the amniotic portion of the cord, and 

 giving off some branches to the amnion, 

 they are continued to the extremity of the 

 allantoid portion, where they end in an 

 expansion of placental ramifications. The 

 amniotic divisions of these arteries are 

 few, and extremely flexuous ; they are in- 

 cluded between the allantoid layer and the 

 amniotic membrane, within which they 

 may be seen. The placental or chorial 

 divisions, infinitely larger and more numerous, leave the end of the cord, 

 and pass in every direction between the chorion and external lamina of the 

 allantois, beneath which they project. By their anastomoses they form a 

 very rich network, from which proceed the capillary twigs that enter the 

 villosities of the placenta. Observation demonstrates that these twigs do 

 not communicate with the maternal vessels, and that they are continued by 

 venous radicles, the origin of the following vessel. 



The Umbilical Vein commences by these capillary radicles of the 

 placental villosities, which unite between the chorion and amnion to form a 

 network of more voluminous divisions and complexity than that of the 

 arteries. Two principal branches are, finally, the result of the coalescing ; 

 and these soon unite into a single trunk, which accompanies the two 

 arteries in the cord. On reaching the mnbilicus, this, the umbilical vein, 

 bends forward on the inner face of the abdominal parietes, where it is 



POBTION OF ONE OF THE FCETAL VILII, 

 ABOUT TO FORM PART OF THE PLA- 

 CENTA, HIGHLY MAGNIFIED. 



a, a. Its cellular covering ; b, i, b, Its 

 looped vessels ; o, c. Its basis of con- 

 nective tissue. 



