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1922.] Pearl Formation in Indian Pearl Oyster. 217 
class comprises according to my investigations, the majority of 
the larger cyst pearls, the latter of the smaller ones of this des- 
cription, which, as I have indicated above, constitute by far 
the larger proportion of cyst pearls. This conclusion to our 
local researches disposes satisfactorily of certain objections 
levelled at the cestode theory and places the latter in its 
proper perspective ; we see that cestode larvae though less 
frequently the cause of pearl formation than was at first believed, 
us now see how pearl formation proceeds,(a) in cyst pearls 
formed around intrusive foreign bodies; (b) in those with a 
fragment of periostracum as nucleus, and lastly (c) in muscle 
pearls. 
f my earliest experiments made in Galle in 1902, 
have direct and fundamental bearing on this problem. These 
were in respect of the power of the oyster to repair injuries to 
the shell. They resulted in demonstrating that epithelial cells 
are capable, at least over the nacre-secreting area, of an altera- 
tion in the character of their secretive power upon emergency. - 
Thus I found that if a fragment of shell in the centre of the 
valve were removed, exposing the mantle which previously 
had been engaged in secreting nacre, the first repair substance 
formed was not nacre, but a yellow parchment-like material 
apparently identical with periostracum. Only after a stiff 
layer of this was formed, was there a resumption of nacre 
secretion. Now in all the pearls I have examined and notably 
in button pearls formed after the old Chinese method, and 
within recent years refined and extensively employed on a com- 
f 
