CHAPTER XLIV. 



THE UROGENITAL SYSTEM 



From our study of the urogenital system of the chick, we learned 

 that while in birds and mammals, a pronephros, a mesonephros, and a 

 metanephros form, in amphibians the first two forms of nephridic organs 

 alone make their appearance, the mesonephros then remaining as the 

 permanent adult functioning kidney. 



We have already spoken of the nephrotomes, which are also called 

 the intermediate cell mass (Fig. 268). 



Some time before hatching, when the embryo is only about three 

 to four millimeters in length, this intermediate cell mass can be seen as 

 longitudinal thickenings on each side of the notochord. These thicken- 

 ings then form a groove, the lips of which soon fuse to form a tube or 

 duct. This is the pronephric or segmental duct. It is at the anterior 

 end of this pronephric duct (in the region of the second to fourth 

 somites) that the pronephros or head-kidney forms as a ventro-lateral 

 outgrowth. 



At this anterior end of the pronephric duct there are three tiny 

 openings left as the lips of the duct fuse. These three openings become 

 the three pronephric tubules (Figs. 349, 350), and the openings of these 

 tubules into the coelom are called the nephrostomes. 



The nephrostomes become lined with large cilia which produce a 

 current out of the coelom, which current then passes by way of the 

 pronephric duct to the cloaca. 



The pronephros itself becomes quite vascular. In the discussion 

 of the posterior cardinal veins, mention \vas made of the close relation 

 of these veins to the excretory system. It will be remembered that the^ 

 lie along the pronephric ducts. The elongating of the pronephric tubules 

 pushes them upward into the posterior cardinal sinus until the sinus is 

 nearly rilled. This means, of course, that the tubules are really bathed 

 in venous blood. 



At the same time that this occurs, arterial blood is brought from 

 the dorsal aorta to the excretory system by arteries in the form of 

 glomeruli. 



The manner in w T hich the glomeruli form is rather complicated. 

 Opposite the second nephrostome, a fold appears in the splanchnic meso- 

 derm, when the embryo is about 4.5 millimeters long. This fold lies 

 parallel to the pronephros itself, becoming elevated and projecting into 

 the coelom opposite the nephrostomes. Vascular spaces appear in this 



