1922] MADRAS PEARL FISHERIES 135 



enveloping it, wherein it lies quiescent and harmless, giving no 

 further irritation. But in the case of a parasitic larva that arrives 

 in the epithelium in a dying condition, exhausted or perhaps 

 smothered in the secreted fluid poured out by the epethelial cells 

 a different situation is found. Instead of being within a layer of 

 connective tissue, it lies in a depression of the epithelial cells and 

 these act differently from connective tissue cells — with a corre- 

 spondingly divergent result. 



In regard to the second and more numerous class of cyst-pearls, 

 usually however much smaller in size than those of the first class, 

 decalcification shows no definite nucleus other than a tiny amor- 

 phous scrap of brownish refractive substance, similar apparently 

 to periostracum. Rubbel of Marburg has investigated the origin of 

 pearls with a similar form of nucleus, obtained from freshwater 

 mussels. He showed that granules of the same appearance not 

 infrequently appear in the secreting epithelial layer of the mantle. 

 These at times appear to cause an irritation that induces the 

 adjacent cells forthwith to begin the deposit of nacre upon these 

 refractive bodies ; later, by radial division and multiplication, these 

 cells form a minute pearl-sac around each nuclear body, which 

 continues the deposit of concentric layers of nacre and thereby 

 produces a pearl. The same sequence of events occurs in the 

 Indian pearl-oyster eventuating as above stated in the production 

 of the majority of cyst-pearls found in the mantle. The irrita- 

 tion produced is so slight that no shock is experienced and there- 

 fore no periostracal repair substance is deposited prior to the first 

 nacreous layer. 



The third class, muscle pearls, remains for consideration. 

 From their place of origin being invariably close to the insertion 

 of muscles attached to the shell and from the columnar nature of 

 their pseudo-nucleus, we may infer that their initial origin is due to 

 the dislodgment of a tiny particle of hypostracum from the inser- 

 tion surface of the shell, caused by some exceptional strain set up, 

 such as an excessive and sudden contraction of the muscle 

 involved. A particle set loose in this manner causes irritation in 

 the same way that an intrusive foreign body or an unwanted parti- 

 cle of periostracum does and with similar effect : a minute pearl-sac 

 is formed, enveloping the particle, which however in this case 

 begins by secreting columnar hypostracum instead of periostracal 

 membrane or nacre. This is consistent with its natural function. 



