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 some places (Pl. XLI. fig. 32). 
If we try to trace the steps resulting in the condition figured 
on Pl. XLIV. fig. 49 and on Pl. XLI. fig. 31 (¢.e. to survey the 
story of the formation of a double pearl), we may assume that 
they were as follows. As the two neighbouring 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 flattened (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 (P]. XLI. fig. 52). At the periphery 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 efforts of the epithelia 
being represented by mac. and wae.’ in fig. 31. Finally, the 
epithelium ceased to control the deposition of its secretion, and, 
with the shrinkage 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 occurred. 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 from 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 covering. 
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 pearls, there was a triangular 
plug of tissue, which for some time remained attached to the 
degenerated 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 ib went the albuminous fluid, which, being secreted at a much 
greater rate than the lime-salts, was practically devoid of lime 
[55! 
