378 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



The special morphological importance of these arrangements lies in 

 the fact that they demonstrate to us how the peculiar arrangement of 

 the testis and its duct in the Teleostei may have come about in the 

 course of evolution from the more primitive arrangement in which the 

 testis communicates with the opisthonephros through a number of vasa 

 efferentia scattered throughout its length, an arrangement still persisting 

 in such a ganoid as Lepidosteus or Adpenser. The stages in the pro- 

 cess are illustrated in Fig. 163 : the degeneration of the hinder end of 

 the testis to form a sterile portion, the restriction of vasa efferentia to 





„x 





7 



Fig. 163. 



Relations of testis and opisthonephros in fishes. A, Hypothetical primitive condition in which 

 the sperms were shed into the peritoneal cavity and passed out bythe kidney tubules; B, 

 Acipenser in which vasa efferentia are scattered over the length of testis and kidney ; C, Lepidosiren 

 in which vasa efferentia are reduced to a few connected with the sterile hinder part of testis ; D, 

 Pfotopterus in which the vasa efferentia are reduced to a single one at the extreme hind end of testis ; 

 E, Teleost in which a kidney- tubule is no longer recognizable as forming part of the channel from 

 testis towards the exterior, j?, Diffuse gonad ; o.d. Wolffian duct ; p.e, peritoneal epithelium ; 

 T, testis; Tj, functional part of testis; Tj, sterile part of testis. 



this portion^ the final reduction of the series of vasa efferentia to the 

 hindmost one of the series, and the simphfication of the path of the 

 spermatozoa (sterile part of testis, vas efferens, kidney tubule) so as to 

 form a simple short tube leading from the functional part of the testis 

 into the kidney duct. 



The skeleton of the Lung-fishes shows one or two extremely archaic 

 features. Thus the notochord persists throughout life and, although its 

 sheath becomes converted into a cylinder of cartilage as in Elasmo- 



