Development of the Fins of Teleosts. 37 



anterior and posterior to the regularly formed muscles are 

 paired masses of closely packed mesenchyme cells, which 

 form a distinct layer under the epidermis. These masses are 

 undoubtedly derived both from the original mesenchyme of 

 the fin and from the breaking down of the rudimentary buds. 

 The posterior mass of each side differentiates into two 

 muscles, one corresponding to the erectors, the other to the 

 skin-muscles. These become attached to the posterior ray of 

 the fin. A depressor muscle is wanting in the case of this 

 ray. 



Anterior to the first of the regularly formed muscles is a 

 cartilaginous ray-support, and anterior to this are the masses 

 or laminae of undifferentiated cells described above. In the 

 middle plane, anterior to the first cartilage, at the regular 

 distance existing between the other ray-supports, cells aggre- 

 gate, and, later, chondrify, forming another ray-support. 

 That portion of the undifferentiated cell-mass lying between 

 this new cartilage and the one next succeeding it segregates 

 from the rest, and from it muscles corresponding to those of 

 the region of the muscle-buds are developed. This process 

 continues until the usual number of rays and muscles found 

 in the adult fin is reached. In Salmo salar this is fourteen 

 in the dorsal and ten in the anal. The anterior ray-support 

 and its muscles are not so fully developed as the others. 



Each fin is innervated by a series of spinal nerves. The 

 nerves of each fin are connected by a longitudinal commissure, 

 which is a branch of the ramus lateralis vagi. I am unable 

 at present to trace the development of this interesting plexus. 



In late embryonic and in the adult stages the metamerism 

 of the fin corresponds to that of the body of the fish. In 

 earlier stages the fin is more concentrated, as exhibited by 

 the strong convergence of the muscle-buds which enter into 

 it. This varies, however, greatly in different species, and is 

 a matter to which but little importance is to be attached. 



The Ventral Fin. — The ventral fin appears considerably 

 later than the median fins. The first traces of it to be seen 

 are slight aggregations of cells in the body-wall just below 

 the ventral edge of the myotomes which lie in the region of 

 the dorsal fin. About the same time the epidermis covering 

 these parts becomes considerably thickened through multipli- 

 cation of its cells. Before the aggregations of mesenchyme 

 cells become very conspicuous muscle-buds grow out from 

 the anterior ventral angle of each myotome in this region : 

 about six enter the fin-rudiment; those at each end of it 

 projecting very obliquely to the axis of the body, converge 

 towards the middle of the fin. 



