DEVELOPMENT OF THE EMBRYO. 163 



exteriorly by the urethra. It is attached to the abdominal walls by a 

 rounded cord, the urachus, the remains of a portion of the allantois. 



Genito- urinary Apparatus. The Wolffian bodies appear about the 

 thirtieth day, as long hollow tubes running along each side of the primi- 

 tive vertebral column. They are temporary structures, and are sometimes 

 called the primordial kidneys. The Wolffian bodies consist of tubules 

 which run transversely and are lined with epithelium ; internally they 

 become invaginated to receive tufts of blood vessels ; externally they open 

 into a common excretory duct, the duct of the Wolffian body, which unites 

 with the duct of the opposite body, and empties into the intestinal canal 

 at a point opposite the allantois. On the outer side of the Wolffian body 

 there appears another duct, the duct of Miiller, which also opens into the 

 intestine. 



Behind the Wolffian bodies are developed the structures which become 

 either the ovaries or testicles. In the development of the female, the 

 Wolffian bodies and their ducts disappear ; the extremities of the Miillerian 

 ducts dilate and form the fimbriated extremity of the Fallopian tubes, while 

 the lower portions coalesce to form the body of the uterus and vagina, 

 which now separate themselves from the intestine. 



In the development of the male, the Miillerian ducts atrophy, and the 

 ducts of the Wolffian body ultimately form the epididymis and vas 

 deferens. About the seventh month the testicles begin to descend, and 

 by the ninth month have passed through the abdominal ring into the 

 scrotum. 



The Kidneys are developed out of the Wolffian bodies. They consist of 

 little pyramidal lobules, composed of tubules which open at the apex into 

 the pelvis. As they pass outward they become convoluted and cup-shaped 

 at their extremities, receive a tuft of blood vessels, and form the Mal- 

 pighian bodies. 



The ureters are developed from the kidneys, and pass downward to be 

 connected with the bladder. 



The Circulatory Apparatus assumes three different forms at different 

 periods of life, all having reference to the manner in which the embryo 

 receives nutritious matter and is freed of waste products. 



The Vitelline circulation appears first and absorbs nutritious material 

 from the vitellus. It is formed by blood vessels which emerge from the 

 body and ramify over a portion of the vitelline membrane, constituting the 

 area vasculosa. The heart, lying in the median line, gives off two arches 

 which unite to form the abdominal aorta, from which two large arteries 

 are given off, passing into the vascular area ; the venous blood is returned 



