350 Proceedings of Royal Society of Edinburgh. [sess. 
with cloacal mucus yielding calcium carbonate, carbonic acid gas, 
and hydrogen or carburetted hydrogen. 
The lime may then be carried to the secreting surface of the 
oviduct in fowls as a soluble phosphate of lime and soda, as 
calcium chloride, as lime soaps (in combination with fatty acids), 
or even as carbonate. This latter is, however, not so probable. 
It is then secreted along with urea, carbonate of ammonia, 
carbonic acid (in a nascent condition), &c. The nascent carbonic 
acid combining with the lime in presence of the urea (or carbonate 
of ammonia), we have a quantity of carbonate of lime deposited in 
the organic membrane, and the shell is formed of insoluble carbonate 
of lime. 
In certain eggs the carbonate of lime is partially replaced by 
phosphate. 
If this takes place in hens, it is probable that similar processes 
may go on in connection with the formation of carbonate of lime 
by marine animals, which have the sulphate of lime presented to 
them in the presence of chloride of sodium. 
In the case of the crab, sulphate of lime is apparently not 
assimilated, even in the presence of chloride of sodium, and crabs 
which throw off their shells in artificial sea water, in which both 
the above salts are present, but from which chloride of calcium is 
excluded, do not form a new exo-skeleton of carbonate of lime. 
As soon as chloride of calcium is added, although the sulphate be 
withheld, shell formation may go on. 
Shell formation in the crab is somewhat different from egg-shell 
formation in the hen, and occupies an intermediate position between 
such egg-shell formation and bone formation, as the carbonate of 
lime is deposited in the chitinous portion of growing epithelial cells 
in the crab shell. In the egg-shell it is deposited in a material 
quite separated from the shell, whilst in bone, the matrix in which 
the lime is deposited, though separated, is intimately connected 
with small cells, not epithelial in character. 
In the egg-shell the organic and inorganic material are both 
secreted by and then separated from the epithelial cells. In the 
crab shell the organic material (chitin) remains attached to the upper 
part of the epithelial cells, and in this the lime salts are deposited, 
probably by a process of dialysis, whilst in the case of bone the 
