MUSEUM OF COMPARATIVE ZOOLOGY. 131 
is secreted, not by the epidermis, but by cells of dermal origin. 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 Sclachians and of the teeth of lower 
vertebrates, have suggested that the secretion of the epidermal cells 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 
