134 MADRAS FISHERIES BULLETIN [VOL. XVI, 



Some of 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 alteration 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 

 commercial scale by the Japanese, I have found that the nucleus, 

 whether it be a cestode larva, a grain of sand or a spherule of 

 mother-of-pearl (as in the Japanese culture pearls), is not overlaid 

 directly by a nacreous layer, but has interposed between its 

 surface and the eventual layers of nacre, a distinct and well 

 marked deposit of stiff yellow membrane identical with repair 

 periostracum, which indeed it is. It is evident that the intrusion 

 of any body into the ectoderm must affect it in a similar manner to 

 that caused by a direct injury, such as a fracture of the adjacent 

 shell would do; hence the impulse of the cells around the intrusive 

 body is to pour out the primary secretion employed to meet such an 

 eventuality. The inmost layer of such a pearl is invariably of 

 periostracum. Only after the effects of the shock have passed and 

 normal conditions are restored, does the nacre secretion begin to 

 be again deposited. What seems to me to be the expla- 

 nation is that the membrane repair substance is really 

 the conchyolin basis of nacre with the lime salts withheld- 

 In other words, after a shock, the epithelial cells intermit the secre- 

 tion of lime salts, but continue the secretion of conchyolin, thus 

 giving a periostracal appearance to what would normally be a 

 nacreous layer (conchyolin + carbonate of lime). 



Another deduction which I have made from the investigation, 

 is that only dead or dying parasites excite an irritation of the 

 character necessary to induce pearl formation. A living parasite 

 does not irritate the tissues in the same way; indeed it merely 

 induces the formation of a tough connective tissue sheath or cyst 



