316* . THE CHICK. 



embryos with about eight pairs of mesoblastic somites (cf. Pig. 

 110). The duct arises on either side as a ridge-like projection 

 of the mesoblast, immediately beneath the dorsal epiblast, and 

 a little to the outer side of the mesoblastic somites. At the 

 stage mentioned, it lies opposite the three hindmost of the eight 

 somites. 



As the embryo grows, the ridge lengthens, extending slowly 

 forwards, and more rapidly backwards. It also becomes free 

 from the mesoblast ; and in embryos with fourteen pairs of 

 somites, i.e. of about the thirty-sixth hour (cf. Fig. Ill), the 

 Wolffian duct is present along each side of the body as a solid 

 rod of cells, lying free between the epiblast and the mesoblast, 

 and extending from the fourth to the fourteenth somite. 



The rod soon becomes tubular, by acquiring a central lumen ; 

 and at the same time its position becomes changed, the meso- 

 blast growing rapidly, and spreading over the duct, between 

 it and the epiblast. The Wolffian duct (Fig. 129, kc) conse- 

 quently gets driven down into the mesoblast, where it lies, about 

 the level of the dorsal aorfcae, imbedded in the mass of mesoblast, 

 sometimes spoken of as the intermediate cell mass, which projects 

 into the dorsal part of the body cavity as a longitudinal ridge 

 between the somatopleuric and splanchnopleuric folds. 



The Wolffian duct continues its growth backwards during 

 the succeeding stages, and towards the end of the fourth day 

 (cf. Fig. 123, kc) reaches the cloaca, or terminal dilatation of the 

 alimentary canal, and opens into its dorsal surface. 



3. The Wolffian Body. 



The Wolffian body is developed entirely from mesoblast, and 

 agrees closely in its structure and mode of development with 

 that of the frog. In its fully formed condition (Fig. 123, km) it 

 consists of a complicated mass of convoluted Wolffian tubules, 

 each of which commences with a Malpighian body, and opens at 

 its other end into the Wolffian duct. 



The Wolffian body extends along the greater part of the 

 length of the dorsal body-wall, as far back as the thirtieth 

 somite. Its anterior end is imperfectly developed, or even 

 rudimentary from the first, but from about the sixteenth to the 

 thirtieth somite the Wolffian body remains of large size almost 

 up to the time of hatching. 



