THE CEYLON PEARL-OYSTER, 313 



The membrane consisted of the lining epithelia of the two sacs, 

 and a layer of parenchymatous tissue between these two 

 epithelia. 



The epithelia, and even the individual cells of the parenchyma, 

 can be detected in soaie places (PI. XLI. fig. 32). 



If we try to trace the steps resulting in the condition figui-ed 

 on PI. XLIV. fig. 49 and on PI. XLI. fig. 31 {i.e. to survey the 

 stoiy of the formation of a double pearl), we may assvune that 

 they were as follows. As the two xieighbouring pearls, each 

 enclosed in a sac, grew in size, by the addition of fresh layers, 

 they exerted a pressure on the intervening tissues, resulting in 

 reduced circulation and consequent malnutrition which began 

 at the first point of contact and extended outwards. Thus the 

 contiguous surfaces tended to become tiattened (fig. 49), and the 

 intervening tissue, consisting of the epithelia of the two pearl - 

 sacs and a small amount of connective-tissue between them, 

 finally ceased to be functional, died, and was preserved as a 

 yellow membrane (PI. XLI. fig. 32). At the periphexy of the 

 area of contact, where the curvatures of the two pearls diverged 

 and were separated by a wedge-shaped plug of tissue, nacre- 

 secretion continued longer, the last eflbrts of the epithelia 

 being represented by nac. and 7iac.' in fig. 31. Finally, the 

 epithelium ceased to conti'ol the deposition of its secretion, and, 

 with the shi'inkage of the atrophied tissues a space occurred 

 on each side between the nacre and the epithelium, into which 

 an extravasation of organic matter and salts occuri-ed. The 

 salts precipitated themselves in the form of columns or raphides 

 with their bases apparently in or on the epithelia, and con- 

 currently with this precipitation the soluble organic substance 

 became converted into the conchyolin framework between the 

 prisms, analogous to the " calcoglobin " framework of Harting's 

 bodies, derived fi'om egg-albumen when calcium carbonate is 

 precipitated in it. Lastly, the epithelia and intervening con- 

 nective-tissue died and probably underwent irregular calcification, 

 breaking away from the still functional tissues and becoming 

 incorporated in the substance of the pearl. The still functional 

 tissues now formed a single sac surrounding the two pearls, and 

 quickly enveloped them both in a common nacreous coveiing. 



Similar processes can be postulated to account for the condition 

 .shown in PL XLI. fig. 30. Here, between the curvatures of the 

 surfaces of the two contiguous peai'ls, there was a triangular 

 plug of tissue, which for some time remained attached to the 

 degenernted membrane which separated the pearls. Its epithe- 

 lium gave rise before it broke away to granular repair-nacre 

 {rep.nac.) on the right, where the disturbance was presumably 

 least, and to a small amount of columnar repair-substance {col.) 

 on the left. Then it broke away from the degenerated and dead 

 membrane between the pearls and retreated rapidly, exuding 

 as it went the albuminous fluid, which, being secreted at a much 

 greater rate than the lime-salts, was practically devoid of lime 



