66 EDWAED MCCRADY, JR. 



several other respects (distended coelomic cavities, highly 

 elevated embryonic region, excessively dense mesenchyme 

 with retarded vascularization) and so probably should be 

 omitted from this discussion. 



The most anterior pronephric anlage is always found oppo- 

 site the seventh somite. The intermediate mesoderm is un- 

 segmented and, at this level, very sparse. In the earliest 

 specimen (17153) a distinct condensation involving only a 

 few cells is formed in the intermediate mesoderm immediately 

 adjoining and continuous with the somatic layer of the 

 lateral mesoderm. This condensation appears in individual 

 sections as a knob projecting medially from the coelomic 

 lining, but in series it shows itself to be for the most part a 

 ridge. The ridge tends to be thick and round on its free edge 

 (where it is broken away from the somites or dorsal meso- 

 derm) and quite thin on its attached edge, which latter oc- 

 casionally breaks through, leaving the thicker edge as a cord 

 of cells running parallel to the coelomic epithelium. These 

 breaks, however, are small, irregular, far apart, and quite 

 'unrelated to the segmentation of the dorsal mesoderm. 



Beyond the last somite (the tenth) the intermediate meso- 

 derm becomes much more abundant, is still markedly con- 

 densed, but not completely separated from the dorsal unseg- 

 mented mesoderm, and also not from the lateral mesoderm. 

 This is the condition back to the neighborhood of the primi- 

 tive streak where dorsal and intermediate mesoderm become 

 indistinguishable. 



There is thus a continuous condensation of the intermediate 

 mesoderm from the level of the seventh somite to the posterior 

 end of the body, which condensation shows a tendency to 

 separate from the dorsal segmented mesoderm and remain 

 as a ridge on the coelomic lining. In the next specimen to 

 be described this nephogenic ridge will be seen to be the 

 anlage of the wolffian duct and of all the tubules of the pro- 

 nephros and mesonephros. 



Specimen 17139 represents the end of stage 24. Figure 22 

 shows a reconstruction of the nephrogenic ridge in this speci- 

 men from its anterior extremity to the end of the eleventh 



