xix] 



EXCRETORY SYSTEM 



441 



metanephros there is essentially the same structure as in the 

 mesonephros, but not only primary and 'secondary but tertiary 

 tubules are produced, and all these tubules open into a single 

 enlargement of the archinephric duct, which is termed the pelvis 

 of the kidney. This pelvis tends to be separated by a constric- 

 tion from the rest of the archinephric duct, and the neck of the 

 communication between the two constitutes the ureter, and as the 

 animal grows in length the ureter is drawn out into a long tube. 

 (E, Fig. 216). 



FIG. 217. Diagrammatic transverse section of an Elasmobranch embryo in 

 order to show the development of renal and genital organs. 



1. Nerve-cord. 2. Notocbord. 3. Myotome. 4. Dorsal sclerotome. 

 5. Ventral sclerotome. 6. Nephrotome. 7. Splancbnocoel. 



8. Mesonephric tubule. 9. Avcbinephric duct. * 10. Eudiment 



of genital organ. 11. Vas efferens. 12. Dorsal aorta. 



33. Alimentary canal. 



The mesonephros, or rather some of its primary tubules, enter 

 into close connection with the genital organ. The genital cells in 

 Craniata at their first appearance seem to consist of groups of cells, 

 segmentally arranged, and these cells originate from the inner wall 

 of the splanchnocoel just below the point where it communicates 

 with the nephrotome. As they increase in number and bulk the 

 segmental arrangement is obliterated, but from each originally 

 separate mass of cells strings of cells originate which grow inwards 

 into the connective tissue above the splanchnocoel and constitute 

 the genital tubes. These genital tubes meet and connect up with 



