THE ORGANS OP THE MIDDLE GEKM-LAYEE, 



361 



of its material is either directly or indirectly the epithelium of the 

 body-cavity, as it has been possible to prove in many cases — in 

 Cyclostomes, Selachians, Amphibia, and Amniota. 



There are formed, following one another in immediate succession, 

 short transverse tubules (fig. 202 st), which are at one end continuous 

 with the epithelium of the body-cavity, and at the other end, which 

 remains for a long time closed, are joined to the mesonephric duct {wd), 



Tig. 203.— Emliryo of a Dog of 25 days, straightened out and seen from in front, after B:schoff. 



Magnified 5 diameters. 

 d, TnteBtinal tube ; ds, yolk-sac ; al, allantois ; un, mesonephros ; I, the two lobes of the liver, 



with the lumen of the vena omphalomesenterica between them ; ue, ht, anterior and posterior 



extremities ; k, heart ; m, mouth ; au, eye ; g, olfactory pit. 



which runs close to them, but somewhat more laterad. The mesone- 

 phros elongates from before backward and attains a great length on 

 both sides of the mesentery, for it reaches back from the region of 

 the liver nearly^ to the posterior end of the body-cavity ; it acquires 

 a very delicate, regular condition, as the figure of an embryo Dog 

 twenty-five days old shows (fig. 203 un), and can be designated as 

 a comh-sliaped gland, composed of a lateral collecting tube, running 

 lengthwise of the body at a little distance from the mesentery, and, 



