THE CEYLON PEARL-OYSTER. 303 
The prismatic substance is secreted by that part of the epidermis 
apposed to the inner surface of the shell which is nearest to the 
margin of the mantle, and it is clear from the rapidity with 
which the columnar lip and its lappet-like processes are re- 
generated when injured and are added to in growing young shells 
that the characters of this layer are associated with relatively 
rapid secretion. This is significant, in view of the resemblance 
of this layer to some of the rapidly secreted repair-substances 
which replace the nacre under certain abnormal conditions 
(see below). 
Romer’s work (p. 18) gives an interpretation of the nature of 
the prisms, which | have found most useful in helping me to 
interpret my own observations on pearl-formation. According to 
this hypothesis, which was suggested to Romer by my illustrious 
teacher, Prof. Biitschli, each prism is an incomplete spherocrystal, 
the growth of which has been arrested in all directions but one, 
viz. the direction from which the new shell-substance is secreted. 
Romer says (p. 18) :— 
“Dass diese Spharokristalle der einzelnen Prismen so 
unvollstindig ausgebildet sind, rithrt daher, dass gleichzeitig 
und dicht nebeneinander die Anfiinge der einzelnen Prismen 
oder. Spirokristalle gebildet wurden, die bald seitlich 
aufeinander stiessen und sich so gegenseitig in der weiteren 
Ausbildung hemmten ; nur an ihren inneren’ Enden 
vermochten sie einseitig weiter zu wachsen ” *, 
If I may be allowed to state the proposition in shghtly 
different terms, the prismatic shell-substance (and, indeed, if my 
interpretation of the variations of the nacre, normal and patho- 
logical, are correct, the whole of the shell-substance) agrees with 
a spherocrystal in that it is composed of crystalline or crystallised 
substance which can only grow by the apposition of fresh layers 
deposited on a single surface, owing to the matter in solution 
only having access to one surface of the crystalline mass. When 
this surface is the outer surface of a sphere, a body with more or 
less of the characters of a spherocrystal results (e. g. Harting’s 
bodies and Pearls); where it is approximately a plane surface, as 
in the growth of the Molluscan shell, a structure such as the 
* Bitschli in 1908 (6, p. 26) explained his definition of ““ spheerocrystals,’’ more 
especially with reference to the crystal-like prisms of Pinna (and by analogy 
of Margaritifera), as follows :— 
“Wie aus den Darlegungen in meinem Werk von 1898 hervorgeht, verstehe 
ich unter einen solchen nicht ein Aggregat zentrisch angeordneter Kristallnadeln 
oder Einzelkristalle, was zwar die tibliche Anschauung ist, sondern ein 
einheitliches Kristallgebilde, in welchem die besonderen feinsten Struktur- 
verhaltnisse, die auch den gew6hnlichen Kristallen ihre charakteristichen 
Kigenschaften verleihen, nicht entsprechend einer Axe, sondern um ein Zentrum 
radiar orientiert sind. Wenn daher der Radius eines solchen Spharokristalls 
sehr gross wird, und man ein radiales Stiick desselben, weit entfernt von dem 
Zentrum, herausschneidet—und so verhalten sich etwa die Pinnaprismen—so 
muss dieses Stiick sich natiirlich wie ein gewodhnlicher Kristal verhalten ; 
obgleich die von mir gegebene Zuriickfiihrung auf einen Sphiarokristall mit 
erossem Radius ganz zutreffend ist.” 
[45] 
