EXCRETORY ORGANS. 



717 



In the posterior part of the Wolffian body of the Chick the intermediate 

 cell mass becomes very early detached from the peritoneal epithelium, and 

 at a considerably later period breaks up into oval vesicles similar to those of 

 the Reptilia, which form the rudiments of the segmental tubes. 



Secondary and tertiary segmental tubules are formed in the Chick, on the 

 dorsal side of the primary tubules, 

 as direct differentiations of the meso- 

 blast. They open independently into 

 the Wolffian duct. 



In Mammalia the segmental tu- 

 bules (Egli) are formed as solid masses 

 in the same situation as in Birds and 

 Reptiles. It is not known whether 

 they are united with the peritoneal 

 epithelium. They soon become oval 

 vesicles, which develop into complete 

 tubules in the manner already in- 

 dicated. 



After the establishment of 

 the Wolffian body there is formed 

 in both sexes in all the Amniota 

 a duct, which in the female 

 becomes the oviduct, but which 

 is functionless and disappears 

 more or less completely in the 

 male. This duct, in spite of certain peculiarities in its develop- 

 ment, is \vithout doubt homologous with the Miillerian duct of 



FIG. 403. SECTION THROUGH THE 

 EXTERNAL GLOMERULUS OF ONE OF 

 THE ANTERIOR SEGMENTAL TUBES OF 

 AN EMBRYO CHICK OF ABOUT IOO H. 



gl. glomerulus ; ge. peritoneal epi- 

 thelium ; Wd. Wolffian duct ; ao. 

 aorta ; me. mesentery. The segmental 

 tube, and the connection between the 

 external and internal parts of the glo- 

 merulus are not shewn in this figure. 



FIG. 404. SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH 

 GIVE RISE TO THE ANTERIOR PART OF THE MiJLLERIAN DUCT (PRONEPHROS). 

 (After Balfour and Sedgwick. ) 



A is the i ith section of the series. 

 B isth 

 C i8th 

 gri. second groove; gr$. third groove ; ri. second ridge ; wd. Wolffian duct. 



