I'KAKL PRODUCTION. 3 



the history of this inquiry, with, where known, the species of shell-fish on which the 

 observations were made, and an indication of the view held, with more or less 

 justification, as to the nature of the nucleus around which the pearl is formed. 



Omitting the more fanciful views, there are evidently three main methods which 

 have been advanced as explaining the formation of pearls ; and as is not infrequently 

 the case when there are several competing theories, it cannot be said that one only is 

 correct and of universal application and that the others are quite erroneous. The 

 three methods referred to are : (1) The grain-of-sand irritation; (2) the pathological 

 secretion ; and (3) the stimulation caused by the presence of a parasitic worm which 

 acts as a nucleus, around which an epithelial sac deposits successive layers of pearly 

 material. We shall briefly examine each of these views in turn. 



Most of the attempts* at artificial " margarosis " the production of pearls by 

 stimulation of the Mollusc have heen based upon the belief that the nucleus of the 



* There is, however, another artificial method which has been suggested by infection with the 

 parasites which will be discussed below. 



B 2 



