Scharff — On Dohrn 1 s Theories on the Origin of Vertebrates. 25 



are the bases of the whole system of muscles in connexion with the 

 limbs. 



The first traces of cartilage in the pectoral fin appear at its 

 base in the mesoderm tissue, between the two pairs of buds. The 

 spinal nerves supplying the fin are equal in number to the 

 myotomes which are concerned in the formation of its muscles. 



As regards the myotomes lying between the pectoral and pelvic 

 fins, they also develop buds ; but these soon disappear again. The 

 ventral unpaired fin makes its appearance in the same manner as 

 the fins just described, and Dohrn supposes that it also must 

 originally have been a paired fin. Just as in the paired fins, the 

 skeleton consists of a series of simple cartilaginous rays. The 

 dorsal fin also appears to originate in a similar way, the muscle - 



VI. 



b.br. — / \ -h.br. 



Diagrammatic figure of Skeleton of a Branchial Arch in an Elasmobranch 



v. c. = vertebral column. 



ph. = basale or pharyngobranchial. 

 ext. = external rays. 



int. = internal rays. 



m. = middle pieces, 

 b.br. = basibranchial. 

 h. br. = Irypobranchial or copulare. 



buds arising from the dorsal end of the myotomes, but with the 

 difference that there is no separation into outer and inner buds. 

 Hence the dorsal fin is apparently unpaired from its very beginning ; 

 however, Dohrn proposes to try and solve this problem presently. 

 In conclusion, he deals with a few phylogenetic questions regard- 

 ing the origin of the fins. The cause of the ventral union of the 

 paired post-anal fins lies in the non-existence of the intestine in 

 that part. But the history of development tells us that there 

 existed a post-anal portion of the alimentary tract. In the 

 ancestral vertebrate this tract extended to the end of the body, 

 the present anus being a more recent formation. "We can under- 



