Development of the Fins of Teleosts. 41 



afterwards probably incorporated into the lateral muscle- 

 masses. The seventh and succeeding myotomes grow ven- 

 trally and are concerned in forming the ventral muscles of the 

 fish. 



The changes that the fin has undergone are now consider- 

 able. The attachment has constricted considerably, at least 

 in comparison with the free portion, which has become a fan- 

 like expansion. With the absorption of the yolk' the fin is 

 brought to the ventro-lateral surface of the body, and, rotating 

 on its axis, so that the line of attachment instead of being 

 parallel to the axis of the body now makes an angle of about 

 forty-five degrees with it, the anterior extremity is thus 

 brought into a corresponding position with that described in 

 the posterior. The internal changes that have taken place 

 during this time are the differentiation of the central core of 

 cells into cartilage, and of the proximal portions of the super- 

 ficial mesenchyme layer into muscle. It may be regarded as 

 certain that the cells which give rise to these muscles originate 

 from the somatopleuric thickening } and, as is the case with most 

 of the muscles of the ventral fn } are in no wag connected with 

 the myotomes. At first there are but two muscle-masses, a 

 primitive abductor or protractor lying on the outer side of the 

 cartilaginous skeleton and an adductor or retractor on the 

 inner side. A superficial muscle is developed later from a 

 mass of cells lying just within the fin between the deeper 

 abductor and the inner epidermic wall. The superficial pro- 

 tractor or abductor does not appear till much later, and 

 probably arises through delamination from the primitive 

 muscle, though I am not perfectly convinced of this. 



The nerves of this fin are distinguished very early in its 

 development, just as in the ventral, i. e. before any differen- 

 tiation of the tissue has taken place. They arise from the 

 first four spinal roots. The first root corresponds to the 

 second myotome, and its ramus ventralis unites soon with the 

 second nerve to form the hypoglossal. This gives off a branch 

 to the fin-plexus and one to the coraco-hyoid muscle. The 

 arrangement is completed very early in the life-history of the 

 individual, and seems to be quite typical for the Teleosts. 



Recapit ulation. 



The mesodermic structures of the median fins are derived 

 from mesenchyme cells derived from the sklerotome and from 

 muscle-buds, which are outgrowths of either the dorsal or the 

 ventral edge of the myotomes. To a certain extent these 

 fins retain their primitive metamerism, in that each muscle- 



