194 THE DEVELOPMENT OF THE CHICK 
ectoderm, until it reaches the level of the cloaca at about the 
sixtieth hour (30-318). It acquires a narrow lumen anteriorly 
at about the 25s stage; but the remainder is solid. At about 
the sixtieth hour the ends of the ducts fuse with broad lateral 
diverticula of the cloaca, and the lumen extends backwards 
until the duet becomes viable all the way into the cloaca (at 
about seventy-two hours, 35s stage). 
The Mesonephros or Wolffian Body. The mesonephros de- 
velops from the substance of the intermediate cell-mass between 
the thirteenth or fourteenth somites and the thirtieth somite. 
There are slight local differences in the relations of the tubules 
in front and those behind the nineteenth and twentieth somites, 
but in general the tubules may be stated to arise as epithelial 
vesicles derived from the intermediate cell-mass, which become 
transformed into tubules, one end of which unites with the Wolfhan 
duct and the other forms a Malpighian corpuscle in the manner 
described below. It will be seen that the anterior mesonephric 
tubules which are relatively rudimentary and of brief duration 
overlap the posterior pronephric tubules; they may possess neph- 
rostomes, whereas the typical mesonephric tubules formed behind 
them, which constitute the main bulk of the mesonephros, never 
possess peritoneal connections. 
An embryo with 29-30 somites is in a good stage for consid- 
ering the early development of the mesonephric tubules. If 
one examines a section a short distance behind the last somite, 
one finds that the intermediate cell-mass is a narrow neck of 
cells uniting the segmental plate and the lateral plate, and that 
the cells composing it are arranged more or less definitely in a 
dorsal and ventral layer, though some occur between. The 
primordium of the Wolffian duct occurs in the angle between 
the somatic mesoblast and the intermediate cell-mass, and the 
aorta lies in the corresponding angle of the splanchnic mesoblast. 
In the last somite (Fig. 107) one finds two important changes: 
(1) the intermediate cell-mass is much broader owing to multi- 
plication of its cells, and as a consequence the two-layered arrange- 
ment is lost; (2) whereas the cells of the intermediate cell-mass 
in the region of the segmental plate could not be delimited accu- 
rately from either the segmental or lateral plate, it is now easy 
in most sections to mark its boundary on both sides. It now 
constitutes, therefore, a rather well-defined but unorganized mass 
