110 THE EARLY DEVELOPMENT OF MAMMALS. 



duct, which, on account of its relations to the organs is in this type more com- 

 monly spoken of as the Wolffian duct. The circulation of the organ is sinusoidal. 

 Further details are given in the practical part in connection with study of the pig 

 embryos, pages 252 and 306. 



The third of the excretory organs is termed the metamphros, or true kidney. 

 In the mammalian embryo, after the Wolffian body has acquired a considerable 

 development, there, appears a small outgrowth of the Wolffian duct at a point near 

 the junction of the duct with the allantois. It extends dorsad and cephalad (Fig. 

 172, ki), and may be termed the renal evagination. Its blind end expands to be- 

 come the pelvis of the kidney, while its stalk remains narrower and is converted 

 into the ureter, by which the urine is conveyed from the pelvis to the bladder 

 (allantois). By outgrowths of the epithelium of the pelvis the collecting tubules are 

 developed. Around the pelvis appears an envelope of special cells, easily recognized 

 by their darker staining (Fig. 210). These cells are thought to be derived from the 

 nephrotomes of the segments of the renal region, which have lost their epithelial 

 arrangement and have migrated to form the envelope. The cells gather themselves 

 gradually into small clumps, .the number of clumps continuing to increase during a 

 long period. Each clump assumes an epithelial arrangement and acquires a lumen 

 and elongates into a tubular form. The tubule elongates, one end joins a collect- 

 ing tubule, and the lumina of the two structures become continuous. The other 

 end of the tubule remains closed and is converted into the renal corpuscle. The 

 tubule now grows rapidly in length and becomes very irregular in its course; the 

 part which joins the collecting tubule becomes the proximal convoluted tubule; the 

 part which joins the renal corpuscle becomes the distal convoluted tubule, and the 

 middle part between these two becomes the loop of Henle. 



The Urogenital Ducts. 



The genital and excretory organs always develop together and constitute the 

 urogenital system. The genital glands are always distinct, but the primary excre- 

 tory or Wolffian duct, after producing the outgrowth to form the renal anlage (p. 

 309), is transformed into the permanent male duct. The other primary canal is' 

 termed the Mullerian duct. It is exclusively genital, for with its mate it develops 

 the uterine tubes, the uterus, and the vagina, but in the male it becomes vestigial. 



The Wolffian duct is the excretory canal of the mesonephros (p. 109). It 

 extends along the ventral surface of the organ; the mesonephric or Wolffian tubules 

 acquire openings into it, and it itself opens at its caudal end into the base of the 

 allantois. It serves for a time as a true excretory duct, but loses this function 

 when the mesonephros degenerates. It meanwhile acquires a secondary connection 

 with the testis by means of some of the Wolffian tubules in the neigrfborhood of the 

 genital gland. The tubules in question acquire a connection with the sexual 

 cords of the testis and, when the cords become seminiferous tubulef, the Wolffian 

 tubules are ready to conduct the semen to the Wolffian duct (vas defer ens). The 



