THE CEYLON PEARL-OYSTER. 315 



Gulf) sliows this substance formed as tlie 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, leacting 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 mollusc shed into the cavity. 

 It probably corresponds to the " Tlieile des 8chalenepidermis " 

 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 

 Su'bstanz " of Rubbel (34, p. 412). 



The amorphous substance shows little receptivity to stains. 

 It passes over sometimes into columnar substance (PI. XLI. 

 fig. 30, tr.), sometimes into granular repair-siibstance (PI. XL. 

 fig. 29, /r., trJ ; PL XLI. fig. 30, tr.'). It also sometimes inter- 

 grades with a substance resembling the prismatic layer (PI. XLTI. 

 fig. 41). Similar intergi^adations with prismatic sulista-nce were 

 shown in some parts of the preparation from which PI. XL. 

 fig. 29 « is drawn. PI. 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 pi'oportions 

 of calcium cai'bonate ))resent. Thus we have in this pearl, 

 which, owing to the impermeability of the amorphous substance, 

 was imperfectly decalcified, tracing the laj'ers from inside out- 

 wards : (1) a plug of nuclear matter of doubtful origin con- 

 taining well-marked crystals (fig. 40, nu.) — 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 difl:ers from the prismatic 

 substance proper (fig. 42, ^jr.) only in the smaller diameters 

 of its constituent elements, a difference which, in view of the 

 variainlity 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 PI. XLV. fig. 52, col., and PL XLII. 

 fig. 36, col., is equally striking. This is shown in detail in 

 PL XLIII. fig. 43. At vv: is the outer wall of the sj^hjero- 

 crystalline or columnar pseudo-nucleus of the pearl. At nac. is 



