THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 355 



layers; p. 99.) The more cephalic of the two tubules becomes hollow and 

 opens into the ccelom; the more caudal is merely a solid cord of cells. Neither 

 tubule forms any connection with the pronephric duct. At each side of the root 

 of the mesentery a small elevation, which projects into the ccelom, probably 

 represents a rudimentary glomerulus. A glomerulus in the lower Vertebrates, 

 where the pronephros develops to a much greater degree than in Mammals, 

 contains tortuous vessels derived from branches of the aorta (Fig. 306). 

 The mesonephros (p. 359), beginning to develop almost as soon as the pro- 

 nephros and in the same relative position, forms a ridge which projects into the 

 coelom. The pronephric tubules thus become embedded in the mesonephric 

 ridge. 



The pronephric duct begins to develop about the same time as the tubules. 

 It appears as a longitudinal ridge on the outer side of the intermediate cell mass 



Sclerotome Myotome 



Visceral 

 mesoderm 



Pronephric 

 tubule 



FIG. 305. Transverse section of a dog embryo with 19 primitive segments. Bonnet. 

 Section taken through sixth segment. 



at the level of the heart and projects into the space between the mesoderm and 

 ectoderm. The ridge is at first solid but soon acquires a lumen, and gradually 

 extends to the caudal end of the embryo where it bends medially to open into 

 the gut. The origin of the caudal portion of the duct is a matter of dispute. 

 It comes in contact and fuses with the ectoderm, but whether in the higher ani- 

 mals the fusion is secondary or signifies a derivation from the ectoderm has 

 not been determined. When first formed, the entire duct lies on the outer side 

 of the intermediate cell mass, but later becomes embedded in the mesonephric 

 ridge. 



The pronephric tubules are but transient structures and have no functional 

 significance in man and the higher Vertebrates. The ducts, however, remain 

 and become the ducts of the second set of urinary organs, the mesonephroi. 



The significance of the pronephros can be understood only by reference to the conditions 

 in the lower animals. In the latter the pronephros acquires a relatively higher degree of de 



