42 On the Development of the Fins of Teleosts. 



bud may be traced directly into a certain muscle of each 

 segment of the fin. Other muscles are derived from cells 

 which are indistinguishable from mesenchyme cells, and 

 which are in all probability to some extent derived from the 

 same. The segmentation of the extreme anterior portion of 

 the tins is secondary, although in the adult no difference can 

 be seen between the two portions. 



The ventral fins show in the early stages of development 

 traces of a similar metamerism. The buds in this case soon 

 disintegrate, and in the space occupied by them a single 

 muscle-mass develops — the adductor or retractor profundus. 

 The other three muscles of this fin are developed from cells 

 which have arisen from the somatopleure, and perhaps also 

 from the sklerotome. This condition in the Teleosts seems 

 to be a step between the Elasmobranchs and the Amphibia. 

 In Triton a few isolated cells break off from the ventral edge 

 of several myotomes and mingle with the cells of the poste- 

 rior extremity, which are, however, mostly derived from the 

 somatopleure. 



According to Paterson (Quart. Journ. Micr. Sci. vol. xxviii.) 

 the myotomes in the chick take no part in the formation of 

 the muscles of the limbs. Kaestner has cast doubt upon 

 this statement, but it is doubtful whether his grounds for so 

 doing are sufficient. 



The pectoral fin is derived entirely from somatopleuric 

 cells. The muscle-buds of this region are greatly modified 

 and take part in the formation of the coraco-hyoid muscle. 



I wish to postpone the full discussion of the meaning of 

 this diversity in the origin of the muscles until some obser- 

 vations on other forms are completed. It is in all probability 

 to be referred to delay in the differentiation of the component 

 parts of the fin until they take up their position within it. 

 In other words, instead of so much connective and skeletal 

 tissue and so much muscle being contributed to the fin, it 

 receives cells which still retain the potentiality to become 

 any of these, and their position with regard to surrounding 

 cells rather than their origin determines their ultimate fate. 



This work, undertaken at the suggestion of Prof. M. 

 Nussbaum, was carried on partly in the Anatomical Institute 

 in Bonn and partly in the Biological Laboratory of the Johns 

 Hopkins University. 



