96 Drs. Kelaart and Mobiiis on the Natural History of 



latter year, in which the same opinion is expressed. This theory 

 of Sir Everard Home's was contradicted by Von Baer, in a paper 

 pubhshed in Meckel's 'Archiv' as early as 1830; and it ap- 

 pears to be untenable as regards the formation of pearls from 

 abortive ova in the ovaries, but a modification of it is brought 

 forward with great appearance of probability by Dr. Kelaart, 

 who says, " that the ova which escape through the distended 

 coats of an overgrown ovarium, and are imbedded in the inter- 

 stices of the mantle, become nuclei of pearls formed in this 

 situation." In support of this view he states that he has " re- 

 peatedly examined seed, or young pearls, in process of formation; 

 and with a magnifying power of ^-inch lens, was able to see 

 distinctly the outlines of two or three ova through the first or 

 superficial layer of nacre, surrounded by groups of ova. It can 

 be readily understood how an overcharged ovarium will, by some 

 accident or spontaneous evolution, have its coats ruptured, allow- 

 ing the ova to escape and become inserted in the contiguous 

 attenuated parts of the mantle. As pearls are more usually found 

 imbedded in the mantle near the hinge, the most likely place 

 where the ovarium is liable to rupture, I consider this very con- 

 clusive of the new theory I have here proposed*." 



(Dr. Kelaart also thinks it possible that the siliceous skeletons 

 of the Diatomacese, which constitute a great part of the food of 

 the Pearl Oystei-, may sometimes, by escaping through the coats 

 of the stomach or otherwise, get into the interstices of the man- 

 tle, and there give rise to the production of pearls. He adduces 

 no positive observations in confirmation of this opinion, except 

 that he once, in examining " seedling " pearls, found a Navicula 

 amongst the [escaped ?] ova.) 



The second modern theory above alluded to is founded upon 

 the observation of the fresh-water pearl-bearing Mollusks [Ano- 

 dontai and Uiiiones), and attributes the formation of pearls to 

 the intrusion of parasites into the mantle. Filippif found in 

 some ponds in the park of Racconigi, near Turin, individuals 

 of the common fresh- water Mussel {Anodonta cygnea), the man- 

 tles of which contained sacs with larvae of Distoma duplicatum, 

 Von Baer, and at the same time saw " a corresponding quantity 



* It would appear, however, that Dr. Kelaart thinks it possible that an 

 over-distended ovarium is one of the causes of peai-ls being discharged 

 from the oyster and lost. He adds : " If this be really the case, it will 

 easily account for the singular fact, that a sample of oysters, fished in the 

 month of October, will yield a larger proportion of pearls than a batch of 

 oysters fished from the same bank in the months of April and May of the 

 following year." 



t SulU Origine delle Perle, translated with notes by Dr. Kiichenmeister, 

 Miiller's Archiv, 1856, p. 251 ; and Encore un mot sur la formation des 

 perks, Miiller's Archiv, 1856, p. 490. 



