MUSEUM OF COMrARATIVE ZOOLOGY. 131 



is secreted, not by tlie epidermis, but by cells of dermal oi'igin. Hence 

 it is not enamel in the modern sense of the term, but may better be 

 known by the name of ganoin, the term introduced by Williamson and 

 recently revived by Klaatsch, 



The great difference in physical properties between this ganoin and 

 the underlying layers of the scale, and the striking resemblance which it 

 bears to the enamel of the scales of Selachians and of the teeth of lower 

 vertebrates, have suggested that the secretion of the epidermal colls may 

 have made its way through the very thin layer of dermal tissue sepa- 

 rating the epidermis from the scale. If this were so, the thin layer 

 of sub-epidermal tissue with its rich blood supply would have to be 

 regarded merely as a device for providing an adequate supply of nour- 

 ishment to the epidermis during its work of secretion, — a device not 

 necessary in the case of the selachian spines by reason of the small size 

 of the secreting area, nor in the mammalian tooth, because of the sunken 

 position of the enamel organ in the gum. This theory would also account 

 for the absence of prismatic structure in the layer. 



The condition of the cells of the basal layer of the epidermis, how- 

 ever, makes this hypothesis untenable. There is nothing in their form 

 or appearance to give any ground for comparing them with the cells 

 active in secreting the enamel cap upon the points of the spines in 

 Lepidosteus, or with the cells which in the Selachians secrete the enamel 

 which coats the scales. They are not more elongated than in early 

 stages when the scale is just beginning to form; their nuclei are not 

 larger, nor do they show any difference in staining quality ; in short, it is 

 impossible to look upon them as an enamel organ. (Compare Figure 10 

 with Figures 16, 22, and 23.) 



We are thus compelled to admit that the dermal scleroblasts give rise 

 to three different products : (1) calcareous scale material with animal 

 matrix and included scleroblasts ; (2) ganoin ; and (3) a membrane which 

 Hertwig has called enamel membrane, but which may better be known 

 henceforth as ganoin membrane. 



This membrane is clearly visible in all sections of tissue from which 

 the ganoin has been dissolved away (Plate II. Fig. 16). It is a struc- 

 ture which is entirely distinct from the basement membrane with which 

 it was confounded by Hertwig, and in sections appears thicker and more 

 prominent than the basement membrane (Figs. 16, 22, and 23). 



It appears that we have to do here not with a differentiation of the 

 cells of one layer, but rather with a modification of the function of the 

 same cells at different periods in their history. The only case at all 



