THE MESOMERE (INTERMEDIATE CELL MASS) 221 



ful that it ever functions as an excretory organ, but it attains its 

 maximum development at about the 12 mm. stage. Along with the 

 glomus the elongated and coiled tubes of the pronephros constitute 

 a formidable mass of tissue which projects into the dorsal body cavity 

 and is surrounded largely by peritoneal epithelium. As the pronephric 

 mass expands, it fills the posterior cardinal sinus and is bathed in 

 venous blood. As the lungs enlarge and grow against the pronephros, 

 the splanchnic mesoderm covering the lungs and the pronephros is 

 brought together to form a trap or pocket which is merely a reduced 

 coelomic chamber into which the temporary nephrocoels open. This 

 is the pronephric chamber. It remains open both anteriorly and 

 posteriorly into the lung chamber. A pronephric capsule is formed by 

 an overgrowth of myotomic mesenchyme and an upfolding of somatic 

 mesoderm from the lateral plate. Thus a connective tissue sheath is 

 formed around the embryonic head kidney. 



By the 1 1 mm. body length stage the pronephroi are large and 

 conspicuous bodies with blind tubular outgrowths; but by the 20 mm. 

 stage they have begun to degenerate and neither the tubules at this 

 level nor the related glomi remain. The mesonephric kidney develops 

 rapidly and takes over the increasingly important excretory functions. 



The Mesonephros or Wolffian Body. 



The nephrotomal mass posterior to the pronephros and mesial to 

 the segmental duct gives rise to the mesonephros or Wolffian body 

 which is the functional kidney of the adult frog. It extends from the 

 level of somites VII through XII and begins to develop at the 8 to 

 10 mm. stage. 



In this region, as in the more anterior regions, there are segmental 

 nephrotomes which merge to form a pair of continuous nephrotomal 

 masses from the seventh to the twelfth somites. The mesonephros is 

 therefore of both somatic and splanchnic mesodermal origin. At the 

 level of each of these somites there develop several nephrotomes, each 

 with a separate nephrocoel. The nephrocoels at this level are known 

 as the mesonephric vesicles which become convoluted and constricted 

 so as to give rise to primary, secondary, and tertiary units, in that 

 consecutive order. It is obvious, therefore, that the kidney units of 

 the mesonephros are more complicated than those of the pronephros, 

 from the very beginning of their formation. They are not actually 

 metameric. 



