THE CEYLON PEARL-OYSTER. 323 
central cavity of a hypostracum-pearl may contain granules of 
doubtful origin, as in the case shown in fig, 21, but it is frequently 
quite empty. At times the organic basis of one of these hypo- 
stracum-pearls, when decalcified, shows a tendency to break up 
into segments, especially at its inner surface; the segments in 
such cases probably correspond to the outlines of the original 
secreting-cells ; indeed, in such cases the whole body may have 
an almost cellular appearance, which is not surprising in view of 
the generally accepted theory that the hypostracum arises by 
direct transformation of the muscle-attachment epidermis. 
These hypostracum-pearls shrink, on decalcification, to about 
one-third of their original diameters (Pl. XX XIX. figs. 22 & 23). 
When the tissue is old and defectively preserved, as in this 
reparation and in that shown in fig. 19, they come away from 
the wall of the enclosing sac during decalcification ; but in better- 
preserved material, where the connection between the muscle- 
attachment epithelium and the pearl is maintained, the organic 
basis of the decalcified hypostracum-pearl remains attached to the 
wall of the sac (Pl. XXXYV. fig. 8). Nacreous pearls, on the 
other hand, almost always shrink away from the sac on decalci- 
fication. 
The smallest of these hypostracum-pearls that I observed 
measured about 0°02 mm. in diameter. 
As the muscle-attachment epithelium, in Margaritifera at any 
rate, takes at most a very small part in shell-thickening, the size 
of these hypostracum-pearls is limited by the maximum thickness 
to which hypostracum normally attains. For further growth to 
occur, resulting in the formation of a nacreous muscle-pearl, it is 
necessary for some of the nacre-secreting epidermis to be present 
also (Pl. XXXYV. figs. 8, s., & 9; Pl. XXXVI. fig. 10, sac.). 
Fig. 8, from a specimen given to me by Prof. Herdman, shows 
above a nacreous muscle-pearl and below a hypostracum muscle- 
pearl. Here we have a cyst, which is more or less spherical, and 
contains a large central cavity lined with a substance which is 
indistinguishable from the organic basis of hypostracum. Where 
an epithelium can be detected in the wall of the cyst (m.ep.') it 
possesses all the characters of muscle-attachment epithelium, its 
cells being continuous with the muscle-fibres, m., on the one 
hand, and with the hypostracum, fy., on the other. In some 
‘ases the muscle-attachment epithelium can be traced on all sides 
of the sac; in others, as in fig. 8 and fig. 10, ¢., only at certain 
parts. In still others nosuch epidermis isrecognisable, I think, 
however, it is safe to assume, whether the hypostracum-pearl is 
surrounded by a sac of attachment-epidermis or not, that such a 
pearl can only arise where such a sac is present ; and it is easy 
to detect the epithelium in most of the better-preserved examples 
(figs. 8, 9, & 10, m.ep.). Still, in some of the fairly well-preserved 
preparations I can identify no such epithelium. ‘This is the case 
in Pl. XXXVI. fig. 11. In this example, which is on one of 
Prof. Herdman’s slides the hypostracum- pearl, which measures 
? « ’ 
21% 
