No. 5. — The Development of the Scales of Lepidosteus. By 



W. S. NlCKEKSON.^ 



The scales of Lepidosteus have been studied by several investigators. 

 Their most prominent peculiai'ity is the possession of a hard, smooth, 

 nearly homogeneous outer layer, which in the adult condition lies free 

 to the exterior not covered by other tissue. The question of most 

 immediate interest concerning the scales is the one in regai'd to the 

 source and nature of this outer layer. The view which has been gen- 

 erally held concerning it is that recently maintained by Oscar Hertwig 

 ('79), who believes that it is a true enamel layer derived from the 

 overlying epidermal tissue. 



The more recent studies of Klaatsch ('90) seem to contradict this 

 conclusion by showing that it has a dermal origin. Closely connected 

 with the question concerning this layer, and in part dependent vipou 

 the answer found for it, is that of the relationship existing between the 

 Ganoid scale and the scales of Selachians, Teleosts, and Dipnoi. Addi- 

 tional interest is also given to this inquiry by the fact that it has a 

 bearing upon the question of the relation of scales to teeth. 



The writers who up to this time have investigated the scales of 

 Lepidosteus have, with one exception, had only adult matei'ial for study, 

 and therefore their conclusions regarding the outer layer were neces- 

 sarily drawn chiefly from the physical properties of the finished scale. 

 Since in the adult the tissue has disappeared from the outer surface of 

 the scales, it is evident that the source of the problematical outer layer 

 can be determined only by the study of young fishes in which the scales 

 are in process of formation, and still covered by the tissue which pro- 

 duces them. 



Klaatsch ('90) in his study of material from a single young gar-pike 

 made some very interesting and important observations, which, if con- 

 firmed by subsequent investigations, must overthrow the idea of Hert- 

 wig, that the outer layers of the ganoid scale are directly homologous 

 with the enamel of the teeth of higher vertebrates. 



^ Contributions from the Zoological Laboratory of the Museum of Comparative 

 Zoology, under the direction of E. L. Mark, No. XXXVI. 

 VOL XXIV. — NO. 5. 1 



