THE CEYLON PEARL-OYSTER, 515 
Gulf) shows this substance formed as the first step in the 
development of a new layer of prismatic substance to cover over 
the tube of the worm Leucodore, which has entered between the 
mantle-margin and the shell, as is its wont. Here the mantle- 
margin, reacting to the stimulation of the parasite, has retreated 
and secreted a new “lip” to exclude it. This lip, like the 
normal lip, consists of the prismatic layer of the shell, but 
the irregularly secreted first layers of it consist of amorphous 
substance, containing alveoli in which a scanty supply of calcium 
carbonate was deposited. 
The amorphous substance frequently occurs in the centres 
and around the central cavities of pearls, where it doubtless 
represents the first matter which the molluse shed into the cavity. 
It probably corresponds to the ‘“ Theile des Schalenepidermis ” 
recognised by von Hessling (18, p. 313) in the nuclei of 
pearls, and the ‘“ Kern von Chitinsubstanz” referred to by 
Pagenstecher (30, p. 502), and perhaps to the ‘“ Gelbbrauner 
Substanz” of Rubbel (34, p. 412). 
The amorphous substance shows little receptivity to stains. 
It passes over sometimes into columnar substance (Pl. XLI. 
fig. 30, é.), sometimes into granular repair-substance (Pl. XL. 
fig. 29, t., tr.'; Pl. XLI. fig. 30, ¢r.'). It also sometimes inter- 
grades with a substance resembling the prismatic layer (Pl. XLII. 
fig. 41). Similar intergradations with prismatic substance were 
shown in some parts of the preparation from which Pl. XL. 
fig. 29a is drawn. Pl. XLII. figs. 40-42 are of interest as 
showing all manners of intergradations between amorphous, 
columnar, and prismatic substances, the different structures 
shown being apparently mainly dependent upon the proportions 
of calcium carbonate present. Thus we have in this pearl, 
which, owing to the impermeability of the amorphous substance, 
was imperfectly decalcified, tracing the layers from inside out- 
wards: (1) a plug of nuclear matter of doubtful origin con- 
taining well-marked crystals (fig. 40, ma.)—these are true 
crystals (rhombohedra) ; (2) a ‘layer of amorphous substance, 
passing over into typical simple columnar repair-substance (fig. 
40, col.); (3) numerous layers of amorphous substance (figs. 40, 
41, am.), some layers being quite lime-free, some having scattered 
alveoli containing calcium carbonate, some showing their cavities 
in radial rows, leading up, by transitions, to regular columnar 
substance (figs. 41, 42, col.), which differs from the prismatic 
substance proper (fie. 42, pr.) only in the smaller diameters 
of its constituent elements, a difference which, in view of the 
variability of the sizes of the prisms in the shell itself, is com- 
paratively unimportant. 
Again, the transition from the abnormal repair-substances to 
nacre in the pearl shown in Pl. XLV. fig. 52, col., and Pl. XLII. 
ne. 36, col., is equally striking. This is shown in detail in 
aman fig. 43. At mu. is the outer wall of the sphero- 
Pelee or columnar pseudo-nucleus of the pearl. At nac. is 
[57] 
