112 PEARLS [CH. VIII 



layer, had the power or potentiality of producing all 

 the other layers of the shell. 



The pearls, however, as we have already seen, 

 consist of some or even all of the layers of substance 

 which form the shell. They sometimes consist only of 

 periostracum and are yellow or brown and horny 

 looking in appearance. Most pearls, however, consist 

 of concentric layers of periostracum, prismatic sub- 

 stance and nacre, with another layer present in 

 places— the hypostracum. These layers may alternate 

 irregularly (figs. 6 and 7). 



The yellow-browii granules, referred to above as 

 being akhi to the substance of the periostracum, are 

 to be found hi the outer epithelium (fig. 12 «). 

 Rubbel believes that some of them are dissolved ; 

 others, however, remain and stimulate the epithelium 

 in some way so that they become surrounded with 

 cells directly continuous with the outer epithelium 

 (fig. 12«). There are many points of resemblance 

 here with the efiect of parasites as described by 

 Boutan, but it will be noticed that the granules 

 (internally produced) are the cause, and that they do 

 not lie outside the epithelium. The cells surrounding 

 the granules secrete new substance round this nucleus, 

 and we may already speak of the little body so formed 

 as a young pearl. Gradually the epithelial pearl sac 

 with its contained pearl breaks away from the outer 

 epithelium (fig. 12 &), until finally it is completely 



