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A MANUAL OF ANATOMY 



nephrotome becomes hollow and gives rise to a mesonephric tubule, other- 

 wise known as a segmental tubule. These tubules open by one end into the 

 pronephric duct, now called the mesonephric or Wolffian duct. Their other 

 extremities open into the ccelom by nephrostomata, but these soon become 

 closed. 



The Wolffian duct extends from the region of the heart to the cloaca, into 

 the ventral compartment of which it opens — that is to say, the uro-genital 

 sinus or canal. The Wolffian duct on either eide lies external to the Wolffian 

 body, and the MuUerian duct is on the outer side of the Wolffian duct. These 

 two ducts lie within a fold of mesoderm, called the uro-genital fold, by which 

 they are connected with the outer part of the Wolffian body. Anteriorly the 

 uro-genital folds, right and left, are wide apart, but posteriorly they converge 

 and fuse. By this fusion a dense fold of mesoderm is formed, called the 

 genital cord, which contains the four ducts — namely, the right and left 

 Wolffian ducts and the right and left Miillerian ducts. The Miillerian duct 

 of either side having crossed over the Wolffian duct, the two Miillerian ducts 

 lie close together at the median line within the genital cord, and in this situa- 

 tion they blend to form the rudiment of the uterus and vagina (utricle or 

 uterus masculinus in the male) . 



Each Wolffian tubule presents a saccular dilatation towards its coelomic 

 end, which dilatation is invaginated by a tuft of capillary bloodvessels. 



Miillerian Duct — 

 Wolffian Duct-- 

 Suprarenal 



Testis -4 — !-"=■ 



Mesonephros or 

 Wolffian Body 



Gubernaculum 



Fig. 359. — Mesonephros and Contiguous Structures of the Embryo 

 Pig: Schematic (B. M. Allen). 



This tuft represents a glomerulus, and the saccular part of the tube invaginated 

 by it represents a capsule of Bowman, the glomerulus and capsule of Bowman 

 corresponding to a Malpighian corpuscle of the permanent kidney. The 

 glomerular part of the tube is the secreting part; and the portion opening 

 into the Wolffian duct is the excretory part, which becomes convoluted. 

 The Wolffian tubules just described are known as the primary tubules. Sub- 

 sequently secondary and tertiary tubules are formed. These tubules have 

 no communication with the coelomic cavity. At one end, which is bhnd, 

 they acquire Malpighian corpuscles, and their other ends join the excretory 

 parts of the primary tubules, and so form collecting tubes. 



The mesonephros or Wolffian body persists in Fishes and Amphibia, and 

 constitutes the functional kidney. In Mammals, Birds, and Reptiles it soon 

 atrophies, and is largely replaced by the metanephros or permanent kidney. 

 The fate of the Wolffian body will be afterwards referred to. 



The metanephros (after-kidney), or permanent kidney, is developed from 

 (i) the renal diverticulum, which springs from the dorsal aspect of the caudal 

 or posterior end of the Wolffian duct close to the uro-genital sinus of the 

 cloaca, and probably (2) the metanephric blastema. The renal diverticulum 

 elongates into a tube which grows cephalicwards, lying dorso-mesial to the 

 Wolffian duct. Its cephalic end extends into the intermediate cell-mass, 

 and is surrounded by mesodermic tissue, known as the metanephric blastema, 

 where it undergoes enlargement. The caudal part of the diverticulum repre- 



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ANATOMY 



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