194 THE DEVELOPMENT OF THE CHICK 



ectoderm, until it reaches the level of the cloaca at about the 

 sixtieth hour (30-31 s). It acquires a narrow lumen anteriorly 

 at about the 25 s 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 duct becomes viable all the way into the cloaca (at 

 about seventy-two hours, 35 s 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 w^hich unites with the Wolffian 

 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 l^etween. 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 



