FROM TWELVE TO THIRTY-SIX SOMITES 195 



of cells between the somite and lateral plate, aorta and Wolffian 

 duct; the posterior cardinal vein appears above the Wolffian duct. 



The next change, found to begin in about the twenty-sixth 

 somite, is a condensation of a portion of the cell-mass lying 

 median to and below the Wolffian duct (Fig. 108), rendered evi- 

 dent by the deeper stain in this region; the condensed portion 

 of the original intermediate cell-mass is not, however, sharply 

 separated from the remainder, but shades gradually into it both 

 dorsally and ventrally, so that it can be seen to represent 

 approximately the central part of the original middle plate. In 

 view of its prospective function it may be called the nephrogenous 

 tissue. Following it yet farther forward one finds that it is a 

 continuous cord of cells with alternating denser and less dense 

 portions, until in the twentieth somite (Fig. 109), the denser 

 portions become discrete balls of radially arranged cells. In 

 the eighteenth and seventeenth somites (Fig. 110) these become 

 small thick-walled vesicles, which are situated median and ventral 

 to the duct. Each vesicle is the primordium of a complete 

 mesonephric tubule. Farther developed tubules are found in the 

 fifteenth and sixteenth somites, and it is probable that the 

 nephrogenous tissue forms mesonephric tubules in the four- 

 teenth, thirteenth, and perhaps yet more anterior segments. 



The formation of the tubules proper from the vesicles may 

 be studied satisfactorily in a 35 s embryo (seventy-two hours). 

 In the twenty-third somite of such an embryo the nephrogenous 

 tissue and the nascent tubules lie median to the Wolffian duct 

 and below the median margin of the cardinal vein (Fig. 111). 

 The Wolffian duct is triangular in cross-section w^ith its longest 

 and thinnest side next the coelome. The most advanced vesicle 

 in this region possesses a hollow sprout extending laterally to the 

 Wolffian duct with which it is in close contact; this is the pri- 

 mordium of the tubular part of the mesonephric tubule (cf. Fig. 

 114 A and B). In more anterior somites it is found that such 

 sprouts have fused with the wall of the duct in such a manner that 

 the lumen of the tubule now communicates with that of the duct. 



Simultaneously the median portion of the original vesicle 

 has been transformed into a small Malpighian corpuscle in the 

 following manner: it has first become flattened so that the lumen 

 is reduced to a narrow slit; then this double-layered disc becomes 

 concave with the shallow cavity directed posteriorly and dorsally; 



