THE CEYLOX PEARL-OYSTER. 323 



centnil cavity of a hypo.stracurn-pearl may contain granules of 

 doubtful origin, as in the case shown in fig. 21, Init it is frequently 

 quite empty. At tinies tlie organic basis of one of these hypo- 

 stracum-pearls, when decalcified, shows a tenrlency to break up 

 into segments, especially at its inner surface ; the segments in 

 such cfises probal)ly correspond to the outlines of the ori^-inal 

 secreting-cells ; indeed, in such cases the whole body may Jiave 

 an almost cellular appearance, which is not surjjrising in view of 

 the generally accepted theory that the hypostracum arises by 

 direct transformation of the muscle-attachment epidermis. 



'J'lifse hypostracum-pearls shrink, on decalcification, to about 

 one-third of their original diameters (PI. XXXIX. figs. 22 & 23). 

 When the tissue is old and defectively pi-eserved, as in this 

 preparation and in that shown in fig. 1 9, 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 peai-1 is maintained, the organic 

 basis of the decalcified hypostracum-pearl remains attached to the 

 wall of the sac (PL XXXV. 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 oljserved 

 measured about 0'(J2 mm. in diameter. 



As tlie muscle-attachment epithelium, in ^^arlJal•itifc/J•a 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 (PI. XXXV. figs. 8, s., & 9 ; PI. 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 h3-postracum. 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., cm the one 

 hand, and with the hypostracum, hi/., on the other. In f-»me 

 cases the muscle-attachment epithelium can be traced on all sides 

 of the sac ; in others, as in fig. 8 and fig. 10, c, only at certain 

 parts. In still others no such epidermis is recognisable. 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, ct 10, m.ep.). Still, in some of the fairly well-preserved 

 preparations I can identify no such epithelium. This is the case 

 in PI. XXXVI. fig. 11. In this exanqjle, which is on one of 

 Prof, llerdman's slides, the hvpostraeum-pearl, which measures 



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