UMBILICAL CORD. 



309 



false knots. (True knots, tied by the intrauterine movements of the 

 embryo, are very rare.) There are no lymphatic vessels or capillaries 



_-Decidual cells. 



^ t **-- Svncvtium. 



&-X-S.? * 



-*M ^t 



Connective tissue. 





Cell knots. 



FIG. 354. FROM A CROSS SECTION OF A MATURE HUMAN PLACENTA. X 260. 



in the cord and the vessels do not anastomose. The arteries contain 

 many muscle fibers but very little elastic tissue and they are usually found 



FIG. 355. CROSS SECTION OF UMBILICAL CORDS. A, X 20. from an embryo of 2 mos. ; B, X 3, at birth. 

 al., Allantois; art., artery; coe., coelom^v., vein; y. s., yolk stalk. 



collapsed in sections; their contraction is of interest since nerves have been 

 traced into the cord for only a very short distance. The vein generally 

 remains open. 



