316 DR. n. LYSTER JAMESON ON 



shown the norraal nacre of the pearl. The first-formed layers of 

 naci"e are incomplete, passing over into this area of repair- 

 snbstance, 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 (gr.), 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 efix)rt to close 

 an opening to the exterior. In such cases it may be secreted so 

 copiously that a tough leathery skin i-esults, with little or no lime- 

 salts in it (PI. XL. fig. 29, am.). It is likewise secreted in layers 

 when a break occurs in the nacre-secretion of a peai-l or of the 

 shell, owing to a pathological extravasation of cellular matter 

 (PI. XXXVIII. 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 ovit 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 

 oi-ganic substance when the latter is called upon to make a 

 special efi"ort 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 seci'etion 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 

 PI. XXXIX. figs. 20 & 21, and PI. XL. figs. 24, 26, & 27 ;_ figs. 20, 

 24, & 27 showing particularly well its continuity and intergi'a- 

 dation with the organic basis of the columnar repair-substance. 

 In the centre of a pearl it may contain, in addition to the 



