1888 - 89 .] Mr Irvine & Dr Woodhead on Carbonate of Lime. 333 
being converted into chitin, and then being shed as it were, whilst in 
the human or other animal skin the cells, as they are gradually 
pushed away from the parent cells, become horny, are separated, and 
so form the cuticle. This is a point of some importance, for the actual 
and direct connection of the chitinous disc is not severed for some 
time, and in that time the chitin becomes impregnated to a lesser or 
greater extent with calcareous particles, the spaces in which the par- 
ticles are found being somewhat irregular. This irregularity is con- 
tinued as the chitin with its lime is continued outwards. It will be 
remembered that in the oviduct of the hen the lining epithelium of 
the secreting follicles and the superficial epithelium contained lime 
in the form of minute granules, that these epithelial cells were 
extremely active, and that lime and organic matter were being 
thrown out on to the surface simultaneously. Exactly the same 
thing takes place in this secreting layer covering the crab; the 
epithelial cells perform a double function — the free end of the cell, 
instead of forming a separate secretion which could be removed at 
once, is converted into a chitinous material. The lower and more 
active protoplasmic portion of the cell separates the lime from the 
salts brought to it by the blood and lymph, and deposits it in the 
chitinous end of the cell, principally in the form of a carbonate but 
partially as a phosphate. 
We have already indicated the process by which, we believe, 
this occurs (Proc. Roy. Soc. Edin., vol. xv. p. 314). The chitinous 
shell, as age advances, becomes thicker and thicker, and as each 
layer is formed it becomes impregnated with carbonate of lime, 
and is then pushed outwards (see also Schmidt, Scientific Memoirs , 
vol. v. p. 10). 
It is an exceedingly interesting fact in connection with this method 
of formation of chitin, that in most cases where there is a deposit 
of a large amount of matrix in more or less direct continuity with 
the secreting cell, and in close contact with the fluids of the body 
(blood and lymph), there is always a larger amount of phosphate of 
lime in the calcareous infiltration than when the matrix in which 
the lime is deposited is actually separated from the secreting cells. 
We shall have again to refer to this. 
§ 4. The subject of the absorption of lime salts, and their elabora- 
tion as carbonate by marine animals, resolves itself into the question 
