442 INTRODUCTION TO CRANIATA [CII. 



a series of tubular outgrowths from some of the primary nephro- 

 tomes of the mesonephros, which constitute what are called the 

 vasa efferentia. 



In the male the genital tubes persist throughout life and form 

 the seminal tubes which make up the testes, and the ripe 

 spermatozoa find their way out through the vasa efferentia, the 

 mesonephric tubules and the archinephric duct, and this latter thus 

 constitutes a vas deferens. In the female the vasa efferentia 

 disappear, the genital tubes termed egg-tubes remain solid and 

 their terminal cells enlarge and form ova, which are thus left 

 isolated in the centre of a mass of connective tissue called the 

 ovarian stroma. At the period of ripeness these eggs swell so 

 much as to burst through this stroma into the splanclmocoel, 

 whence they escape by pores situated near the anus (Eels and 

 Lampreys) or by tubes, the oviducts opening in the same position, 

 which seem to have been developed from grooves in the roof of the 

 splanclmocoel leading to the pores. The oviducts of Elasrno- 

 branchii are peculiar in that they seem to be formed by a splitting of 

 the archinephric duct (F, G, H, Fig. 216). 



The mesonephros is thus distinguished from the metanephros by 

 being the sexual region of the kidney. In most cases it persists 

 throughout life only in the male, vestiges only being found in the 

 female, but in some primitive types of fish, such as the Sturgeon 

 and in Amphibia, mesonephros and metanephros are not clearly 

 differentiated from one another, and a fully developed metanephros 

 with pelvis and ureter is only found in Reptiles, Birds and Mammals. 

 If the above description has been followed it will be evident 

 that the nephrotomes of Craniata correspond closely in position and 

 origin to the gonadic pouches or gonocoeles of Amphioxus and have 

 nothing to do with the excretory organs of that animal. 



They are in fact coelomiducts, and it appears probable that 

 they originally served as outlets for the genital products and 

 correspond to the pores which are formed in the walls of the 

 .gonocoeles of Amphioxus at the period of sexual maturity. They 

 still function as outlets for the spermatozoa in most Craniata, 

 and the reason wliy they have ceased to serve as passages for the 

 eggs is probably to be found in the circumstance that the eggs of 

 all Craniata have become swollen in size owing to the accumulation 

 of yolk so that they are no longer able to pass through such narrow 

 passages as the kidney tubules afford, and hence have to find their 

 way out in another manner. 



