THE ORGANS OF THE MIDDLE GERM-LAYER. 



361 



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

 body-cavity, as it has been possible to prove ~5T 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), 



Fig. 203. Embryo of a Dog of 25 days, straightened out and seen from in front, after BISCHOFF. 



Magnified 5 diameters. 

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



with the lumen of the vena omphalomesenterica between them ; ve, he, anterior and posterior 



extremities ; h, 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 comb-shaped gland, composed of a lateral collecting tube, running 

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



