316 DR. H. LYSTER JAMESON ON 
shown the normal nacre of the pearl. The first-formed layers of 
nacre are incomplete, passing over into this area of repair- 
substance, and all stages of transition may be seen, corresponding 
to the gradually increasing control exercised by the secreting 
epithelium. 
At first, amorphous substance (am.), alveolar in places, was 
secreted, no doubt with irregular crystallised bodies in the alveoli, 
some of which are actually preserved in the preparation, owing 
to incomplete decalcification. Peripherally this gave place to 
columnar substance (col.), which acquired a finely alveolar structure, 
and passed over, through granular repair-nacre (g7.), Into normal 
nacre (nac.'), the layers of the conchyolin of which gradually merge 
into the horizontal markings of the granular substance. The 
amorphous substance in this preparation varies from layer to 
layer in the degree to which it is alveolar ; at some places it 
might better be described as coarsely columnar substance. 
Amorphous substance seems to be the first product where the 
shell is perforated and the mantle makes a sudden effort to close 
an opening to the exterior. In such cases it may be secreted so 
copiously that a tough leathery skin results, with little or no lime- 
salts in it (Pl. XL. fig.29, am.). It is hkewise secreted in layers 
when a break occurs in the nacre-secretion of a pearl or of the 
shell, owing to a pathological extravasation of cellular matter 
(Pl. XX XVIII. fig. 17, am., am.’). These facts suggest that the 
organic basis of the shell is the constituent the secretion of which 
varies in quantity, the secreting-tissues (perhaps the granular 
subepithelial parenchyma in Margaritifera) containing a 
reserve of this material which can be poured out profusely when 
the shell is injured. It would seem that the lime-salts, on the 
other hand, are secreted more regularly, so that the mechanism 
for furnishing these cannot keep pace with that which yields the 
organic substance when the latter is called upon to make a 
special effort to repair damage. The resemblance of the inner 
layers of the periostracum (in forms with a thick periostracum) 
to amorphous repair-substance may perhaps be explained by 
postulating the absence or inhibition of the lime-secreting 
mechanism in the underlying tissues. 
That the secretion of calcium carbonate could not keep pace 
with that of the organic substance, when the latter is produced 
in large quantities, is easy to understand in view of the very 
small proportion of CaO in the blood of Mollusca, and indeed of 
all invertebrates that have been investigated. According to 
Griffiths (quoted by Biitschli, 6, p. 62), the CaO in the blood of 
a number of bivalves examined varied from 0°032 per cent. in 
Anodonta to 0:067 per cent. in Mytilus. 
Amorphous substance is seen in the pseudo-nuclei of pearls in 
P). XX XIX. figs. 20 & 21, and Pl. XL. figs. 24, 26, & 27; figs. 20, 
24, & 27 showing particularly well its continuity and intergra- 
dation with the organic basis of the columnar repair-substance. 
In the centre of a pearl it may contain, in addition to the 
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