128 BULLETIN OF THE 



The scleroblasts overlying the scale about the bases of the spines do 

 not cease their secretive activity when the spines are completed, but 

 their secretion continues to be employed gradually to thicken the scale 

 by additions to its upper surface. The amount of material so added 

 is very little in the central part of the scale, but toward the margins, 

 where growth is still taking place, and where the greater number of 

 spines are foi-med, it is considerable (Plate II. Fig. 13). This results in 

 the basal ends of the spines being surrounded and incorporated in the 

 outer layers of the scale (Plate IV. Figs. 24, 25). These outer layers 

 also contain enclosed cells, and are composed of exactly the same mate- 

 rial as the layers immediately underlying them. 



It is this outer part of the scale which Klaatsch believes to be the 

 enamel layer of Hertwig and other authors, and which lie calls ganoin. 

 He says (p. 141) : " Sie [die Ganoinschicht"] entsteht im Anschluss an 

 die Zahnbildung auf der Schuppe ; sie ist eine direct Fortsetzung des 

 Zahnbeins der kleinen Schuppenzahne." He also says (p. 132): "Die 

 Substanz der Ganoinschicht stimmt in ihrer homogenen Beschaffenheit 

 mit dem Dentin der Zahnchen liberein." 



In this I believe that Klaatsch is in eri'or. It is inconceivable to me 

 that any one who had seen ground sections of the ganoin under the 

 microscope could for a moment confound with it the dentine of the 

 spines. Moreover the figure of ganoin given by Klaatsch (Tafel VII. 

 Fig. 6) represents a condition quite different in appearance from that 

 presented by the true ganoin. Furthermore, as shown by the table 

 already given (page 117), I found no trace of the ganoin layer on the 

 scales of a fish 289 mm. long while the fish from which Klaatsch's mate- 

 rial came was only 180 mm. long. Eeissner ('59, p. 2G0) says concerning 

 the spines " ihre Insertionsstellen unmittelbar unter dem Schmelz lie- 

 ,gen." The presence in this outer layer of enclosed osteoblasts, which are 

 absent from the layer described by Reissner and Hertwig, the fact that 

 it is not destroyed by acid, and its optical properties, all give convincing 

 proof that the material secreted " im Anschluss an die Zahnbildung 

 auf der Schuppe " is not the layer described by Hertwig and others 

 as enamel. If then, as I believe, Klaatsch did not see the layer in 

 question, his claim in regard to its origin can have no weight, and the 

 question of its source remains where Hertwig left it.^ 



1 It is only fair to state, however, that both the 17.5 cm. and the 10 cm. gar- 

 pikes which I studied were killed in acids, so that I liave no imdecalcified material 

 of the same size as that which Klaatsch studied, for a perfect control of his state- 

 ment about the ganoin layer. 



