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AN INTRODUCTION TO ZOOLOGY 



conveyed to the modified urinary tubules, and thence to the meso- 

 nepheric duct. As has been noted above, this duct, functioning as 

 a vas deferens, is a thick-walled, greatly- coiled tube lying on the 

 ventral side of the mesonephros. The cranial mesonephric tubules 

 secrete a nutritive fluid in which the sperms live. The semen, as 

 this fluid with its contained sperms is termed, is probably stored in 

 the vesicula seminalis until required for use. 



For convenience in reference we may tabulate the relation 

 between the embryonic and adult excretory ducts in the following 

 manner : 



EMBRYONIC EXCRETORY STRUCTURES AND THEIR FA.TE IN THE ADULT. 



The histological structure of the kidneys in Scyllium is 

 essentially similar to that in the kidneys of Rana. They are com- 

 posed of a mass of urinary tubules, and differentiated into glandular 

 and collecting parts, and possessing Malpighian bodies. The primary 

 uriniferous tubules also open into the dorsal coelomic space by means 

 of nephrostomes. The tubules are surrounded by a small amount of 

 connective tissue in which run enlarged blood-vessels, the sinusoids 

 fed by the renal portal vein and the renal arteries. 



