SECRETION OF NACRE. 283 



will depend upon their age, live in deep water beyond 

 the reach of man." * 



This conclusion of the comparative anatomist is not 

 acquiesced in by writers on natural history. 



" This," observes Mr Johnston, " is far from being 

 true. I will not deny that the fact may be, in not a 

 few instances, as stated by Sir E. Home for the ovum 

 may accidentally fall into a situation where it shall be- 

 come a source of irritation, like any other extraneous 

 substance ; but that it is often the case is contradicted 

 by numerous observations, and by the true theory of 

 the formation of pearls. Professor Baer of Konigsberg, 

 aware of Home's theory, undertook an investigation of 

 it in the fresh-water mussels of Germany; and the re- 

 sult was, that he never met with pearls in the ovaries, 

 liver, kidney, or any of the internal organs. The pearls 

 were always situated either in or under the skin of the 

 back, where it is close to the shell." 



This is certain the animals producing pearls possess 

 the power of covering with concentric layers of nacre 

 portions of their shells needing to be strengthened, or 

 objects introduced accidentally or by design. The 

 Chinese have long practised the art of stimulating the 

 secretion of nacre by the introduction of mother-of- 

 pearl roughly filed into a plano-convex form, like the 

 top of a mother-of-pearl button. Mr Gray, observing 

 in the British Museum some very fine pearls thus pro- 

 duced, tried the experiment of introducing similar pieces 

 of mother-of-pearl into the shell of the Anodonta Cyg- 

 neus and Unio Pictorum. " If," he observes, " this plan 

 succeeds which I have scarcely any doubt it will we 

 shall be able to produce any quantity of as fine pearls 

 as can be procured from abroad." 



It is to be regretted that the result of this experiment 

 is not known. As it deserves to be repeated, we give 

 Mr Gray's account of the modus operandi: "I found 

 * 'Comparative Anatomy,' vol. v. p. 313. 



