126 BULLETIN OF THE 



face of the scale except at one point, where a small openmg persists 

 through the side of the base of the spine at its junction with the scale 

 (Plate I. Fig. 7, and Plate IV. Fig. 26). 



I have as yet said nothing concerning an enamel layer upon the 

 sphies, because it could not be observed in any material of the stages 

 described. Both Hertwig and Klaatsch assert that the points of the spines 

 are covered by a cap of enamel. Eeissner says (p. 260) : " Die ganzen 

 Stacheln scheinen iibrigens audi einen dlinnen Ueberzug von Schmelz 

 zu besitzen." In scales which have reached the condition in which 

 spines are present the thickness and hardness are such as to make section 

 cutting impracticable, unless the tissue is first decalcified. In sections 

 of decalcified scales no trace of an enamel cap is to be seen, nor is any 

 space left between the point of the spine and the base of the overlying 

 epidermis to indicate that anything has been lost. I was for some time 

 inclined to believe that both Hertwig and Klaatsch were mistaken in 

 asserting the presence of an enamel cap. In spines, too, wdiich were 

 broken off from scales which had been treated with caustic potash to 

 remove the fleshy matter, there was evidently no enamel present. Such 

 a spine is shown in Plate II. Fig. 11. Its cavity is seen to be con- 

 tinued up into the tip as a dendritic system of fine tubules (Den- 

 tinrohrchen of Hertwdg), whose finest branches can be traced quite to 

 the surface, and so preclude the possibility of an enamel cap being 

 present. 



I succeeded in learning the true condition only when, by the aid of a 

 low power of the microscope, I watched the caustic potash slowly eat 

 away the tissue from the surface of the spine-bearing scale. As the 

 epidermal tissue became clear and began to be dissolved, there could be 

 seen supported in it minute scattered caps unconnected with the scale 

 or the spines. With a little search, however, these caps wei'e found 

 in some cases resting directly over the points of spines, but raised 

 slightly from them (Plate IV. Fig. 27). Similar conical caps were also 

 found in the bottom of the watch-glass after the soft tissues had been 

 entirely destroyed. Several of these loose caps were picked up and 

 transferred to glass slides. On treating them with weak acid (2% HCl) 

 their points quickly dissolved away, leaving the cap truncate (Plate II. 

 Fig. 12, a, h). The rest not only was not dissolved, but it showed no 

 change when stronger acid (10% HCl) was added. From this it would 

 seem that the only part of the spine which can be considered to be 

 enamel is the small apex which is dissolved away by the acid, or else 

 that the basal cells of the epidermis first secrete a substance which is 



