94. HERBERT E. DURHAM. 
tissue, and he contends that they arise endogenously in the 
outermost layer of epithelial cells and travel inwards; so 
also with the pigment-cells. The evidence he brings for- 
ward, based largely on the relative numbers of cells in the 
different layers, is far from conclusive. His descriptions 
and figures are complicated by spherules of various staining 
powers, which in many cases seem to be food-yolk granules 
present, owing to the larval condition of the material he 
worked with—a condition which may account for the simi- 
larity he noticed between some of his “ leucocytoid”’ cells 
and epithelial cells which had not yet attained to a complete 
individuality. 
It is rather odd that the outermost layer of epithelial 
cells should be the active ones which give rise to the wander- 
cells (“‘ Leucocytoiden und Perigenzellen”), as he maintains, 
his reason being that it is only in the outermost layer that 
“endogenous formation” is seen to occur; is not this appear- 
ance more probably accounted for by the decreased vital ac- 
tivity of the cells of the outermost layer having become unable 
to resist the invasion of the outwandering cells, which have 
consequently gained admittance to their interior? Moreover 
_the guanin compound, as described by Leydig, is undoubtedly 
an excretory product, and there is considerable evidence! that 
the “‘melanin” granules are a product arising in the disin- 
1 Sheridan Delépine (25), in a preliminary report of his work, considers 
that ‘melanin is elaborated in certain epithelial cells like other products of 
glandular activity out of plasma, and is not a derivative of hemoglobin ;” and 
that “an antecedent, a variety, or a derivative of melanin” passes to the 
lymphatics of the skin, and probably has something to do with the production 
of hemoglobin. It is certain that pigment-cells exist in the cutis and amongst 
the epidermal cells, and pigment-granules within the latter; it is, therefore, 
a matter of interpretation as to which way the pigment is passing, for 
Delépine states that the process of solution of the melanin, and its absorption 
by the lymphatics of the true skin, are not visible. In frogs and newts there 
can be no doubt that “melanin” is got rid of through the medium of the 
skin, and is therefore to be considered as effete material; following List and 
others, it appears to be formed from hemoglobin, the ferruginous portion of 
which may be used again for the formation of new hemoglobin in the tissues 
which produce that substance, as Delépine supposes. 
