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 Wolfhan 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 35s embryo (seventy-two hours). 
In the twenty-third somite of such an embryo the nephrogenous 
tissue and the nascent tubules he lateral to the Wolffian duct 
and below the median margin of the cardinal vein (Fig. 111). 
The Wolffian duct is triangular in cross-section with its longest 
and thinnest side next the ccelome. 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 (ef. 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; 
