THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 389 



velopment than in the higher forms. The tubules are segmentally arranged and are present 

 in many segments of the body. They open at their outer ends into the ducts, and at their 

 inner ends into the ccelom through ciliated funnel-shaped mouths or nephrostomes. Little 

 masses of mesoderm, containing tortuous vessels derived from branches of the aorta, form 

 glomeruli which project into the ccelom. Waste products are removed from the blood 

 through the agency of the glomeruli and are collected in the ccelom. They are then taken up 

 by the pronephric tubules and carried away by the ducts. In some of the Round Worms 

 there is not even a longitudinal duct, but the tubules open directly on the outer surface of 

 the body. In the lowest Fishes all the tubules on each side open into a longitudinal duct 

 which opens into the cloaca. In these lower forms of animal life the pronephroi constitute 

 the permanent urinary apparatus. In the ascending scale the mesonephroi appear (higher 



Nch. 



X-N L^J r^\ 1 



Pron. t. 



Glom. 



FIG. 344. Diagram of the pronephric system in an amphibian. Bonnet. 



Ccel., Ccelom; Glom., glomerulus, containing ramifications of a branch of the aorta; 



Nch., notochord; Pron. t., pronephric tubule. 



Fishes, Amphibia) and assume the function of carrying off waste products. The prone- 

 phroi also develop, but to a lesser degree. Still higher in the scale (Reptiles, Birds, Mam- 

 mals) the kidneys (metanephroi) appear and the mesonephroi lose their functional sig- 

 nificance. But even in the very highest Mammals the pronephroi appear, in a very rudimen- 

 tary form, in each individual in the earliest embryonic stages, thus repeating the ancestral 

 history. 



THE MESONEPHROS. 



The mesonephroi, which constitute the second set of urinary organs, appear 

 in embryos of 2.6-3.0 mm., immediately following the pronephroi. They be- 

 gin to develop just caudal to the pronephric tubules and in the same relative 

 position as the latter, that is, in the intermediate cell mass. Condensations* 

 appear in the mesenchyme and become more or less tortuous. At their inner 

 ends they form secondary connections with the mesothelium and at their outer 

 ends they join the pronephric duct which now becomes the mesonephric (or 

 Wolffian) duct. The cells acquire an epithelial character, lumina appear, 

 and the tortuous mesenchymal condensations thus become true tubules. Their 

 connections with the mesothelium soon disappear (Fig. 345). 



*The term "condensation" is here used to mean increased density of tissue due mainly to 

 proliferation of cells. 



