232 PEARLS. 



*' From these facts, I have been led to conclude that a pearl 

 is formed upon the external surface of an ovum ; which, 

 having been blighted, does not pass with the others into 

 the oviduct, but remains attached to its pedicle in the ova- 

 rium, and in the following season receives a coat of nacre 

 at the same time that the internal surface of the shell re- 

 ceives its annual supply. This conclusion," he adds, "is 

 verified by some pearls being spherical ; others having a 

 pyramidal form, from the pedicle having received a coat 

 of nacre as well as the ovum." * This conclusion, however, 

 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 become a source of irritation like any other extraneous 

 substance ; but that this is often the case is contradicted by 

 numerous observations, and by the true theory of the for- 

 mation of pearls. Professor Baer, of Koenigsberg, aware of 

 Home's theory, undertook an investigation of it in the fresh- 

 water mussels of Germany, and the result was, that he never 

 met with pearls either 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. -|- 



I shall conclude what I have to say concerning pearls with 

 the following extract from the paper of Mr. Gray quoted 

 in tlie preceding page : — " The pearls are usually of the 

 colour of the part of the shell to which they are attached. 

 I have observed them white, rose-coloured, purple, and 

 black ; and they are said to be sometimes of a green colour.;}: 

 They have also been found of two colours ; that is, white 

 with a dark nucleus, which is occasioned by their being first 

 formed on the dark margin of the shell before it is covered 

 with the white and pearly coat of the disk, which, when it 

 becomes extended over them and the margin, gives them 

 that appearance. 



" Pearls vary greatly in their transparency. The pink 

 are the most transparent ; and in this particular they agree 

 with the internal coat of the shell from which they are 



* Comp. Anat. v. 302 ; and Phil. Trans. 1826, p. 339, 

 t Edinb. New Phil. Journ. xiv. 186. There is a very interesting paper on 

 the growth and structure of pearls in the Edinh. Phil. Journal, xi. 39, &c. 

 See also Rcaunnu" in Me'm. de I'Acad. Roy. dcs Sciences, an. 1717, p. 243, &c. 

 I " There are, besides, [in Britain] several sorts of shellfish, among 

 which are mussels, containing pearls often of the best kind, and of every 

 colour ; that is, red, purple, violet, green (prasini), but principally white, as 

 we find in the Venerable Bede's Ecclesiastical History." — Richard of Ci- 

 rencester, trans, p. 28. 



