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 test is and its duct in the Teleostei may have come about in the 

 course of evolution from the more primitive arrangement in which the 



is 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 Acipenser. The stages in the pro- 



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



the test is to form a sterile portion, the restriction of vasa efferentia to 



,A 



>o/v. 



-VL, 



ov 



r 

 V 



^ 



=?-^A^ 



V 



~v>^ 



XI 



E 



/ 



FIG. 163. 



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

 tin- sperms were shed into the peritoneal cavity and passed out by the kidney tubules; l'>, 

 Acifienscr 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, 



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



.i.'st in which a kidney-tubule is no longer recognizable as forming part of the channel from 



inwards the exterior, p, Diffuse gonad ; o.d, Wolffian duct; p.e, peritoneal epithelium; 



; ; TL functional part of testis; T 2 , sterile part of testis. 



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

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

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

 form ji 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 



Thus the notochord persists throughout life and, although its 



'\\ becomes converted into a cylinder of cartilage as in EJasmo- 



