126 BULLETIN OF THE 
face of the scale except at one point, where a small opening 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 
spines, 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 Reissner says (p. 260): * Die ganzen 
Stacheln scheinen übrigens auch einen dünnen 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, which 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- 
tinröhrchen of Hertwig), 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 were 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, b). 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 
