LINNEAN SOCIETY OF LONBON. 



23 



ova, embrvos, and various kinds of parasites or other organic 

 nuclei. I give here, in tabular form, some of the leading names 

 (by no means a complete list) in the history of this enquiry, \Aith, 

 whei-e 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 three main methods 

 which have been advanced as explaining the formation of pearls ; 

 and, as is so often the case where 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 I refer to are : (I) the grain-of-sand irritation ; (2) the 

 pathological secretion ; and (3) the stimulation caused by the pre- 

 sence of a parasitic worm, which acts as a nucleus around which 

 an epithelial sac deposits successive layers of pearly material. 

 Let me say a few words about each of the two last before passing 

 to the first view, the one with which Linnasus was more closely 

 associated. 



