Scharff — On Dohrn's Theories on the Origin of Vertebrates. 33 



looked upon as rudiments of pelvic fins. Moreover, Dohrn advances 

 the hypothesis that the so-called anal fin-muscles in the lampreys 

 may be homologous with the muscles of the pelvic fin of fishes. 



X. — The Origin of the Vertebrate Eye. 1 



According to Dohrn's theory of descent, the tunicate eye must 

 either be looked upon as an entirely new formation, or it must 

 have degenerated from the vertebrate eye to its present condition. 

 It would be difficult, however, to harmonize the first view with the 

 principle of change of function. The solution, then, must be looked 

 for in the second hypothesis. 



In order to find the phylogenetic path of changes from the 

 paired and highly differentiated vertebrate eye to the unpaired 

 and apparently very primitive organ of vision in the tunicates, 

 such complications arose as to compel Dohrn to put off his detailed 

 publication. He proposes, therefore, at present only to give a 

 general outline of his hypothesis. 



The starting-point of Dohrn's inquiry is the development of 

 the eye-muscles. Balfour indicated that the various muscles oftiie 

 eye (rectus externus and internus, sup. and inf., obliquus, sup. and 

 inf.) took their origin from the anterior head- cavities. Marshall 

 completed these investigations and ascertained that the rectus int., 

 sup., and inf., and the obliquus inf. arose from the most anterior 

 head cavity, which had been called praemandibular by Balfour. 

 He also found that the obliq. sup. was formed from the dorsal 

 piece of the second or mandibular cavity, and the rectus ext. even 

 from the third or hyoid cavity. 



During the change of these anterior head-cavities into real 

 muscles, Dohrn ascertained that there was a general agreement 

 with the transformation of the ventral parts of the posterior head- 

 cavities, and not with that of the myotomes. There is no difference 

 observable in the formation of the eye-muscles and the other 

 muscles formed by the most anterior head-cavity. 



If we can prove with certainty, says Dohrn, that the eye- 

 muscles are not body-muscles, and do not therefore correspond to 



1 Ibid., vol. vi. 1885, pp. 432-480. 



SCIEN. PROC. R.D.S. — VOL. VI., PT. I. D 



