PEARL PRODUCTION. 9 



and it was gratifying to find that Professor A. (ii\i;i> in a note* on the subject 

 shortly afterwards book the same view and considered that in the case of Jameson's 

 mussel pearls there is a " passive immigration " of (lie epithelial cells caused by the 

 migrating parasite. 



Just as this section of the report was going to press I received a letter from 

 Dr. Jameson (now on the staff of the Transvaal Technical Institute, Johannesburg) in 

 which he says : "I had never any doubt that it is a true epidermis, but I never got 

 so far as to determine actually by observation whether it arose, as I think you have 

 suggested, by the Trematode carrying in with it a fragment or pocket of epidermis ; 

 or, as I suspected, by means of epidermal or sub-epidermal replacement cells (Ersatz- 

 zellen)." From this it may be gathered that Dr. Jameson would now agree with 

 Giard and Boutan and ourselves that the epithelium of the pearl-sac must be derived 

 directly or indirectly from the epidermis of the mantle. 



The second point in Jameson's account which, from the evidence presented, is not 

 quite satisfactorily settled is the supposed infection of the mussel with parasites by 

 other mollusca Tcq>cs decussatus in France and Cardium edule (the cockle) in the 

 Barrow Channel. So far as regards this case, Jameson's conclusion is based upon the 

 experiment of placing some mussels which he supposed to be free from parasites in 

 a tank with French Tapes which were infected, and examining the mussels from time 

 to time until he found they contained the parasites (Cercaria). Now in such an 

 experiment it is necessary to be quite sure of the material used, to deal with 

 sufficiently large numbers, and to have control experiments. Jameson may have taken 

 these precautions, but it does not appear from his paper. He says of the material : 

 " These mussels, of which I examined a number, were practically without parasites. 

 About one in every five of the largest examples contained a Cercaria, one had two 

 Cercaria?, and one contained a small pearl." This can scarcely be described as free from 

 parasites. He used 70 mussels, and if we take his own figures, one in five, as accurate, 

 then about 14 of these specimens were infected at the beginning of the experiment. 

 We find from his records that he only examined 13 of these mussels (2 after 11 days, 

 6 after 2 months, and 5 after 6|- months), and found 12 of them infected. But it is 

 obvious that that number may have been infected from the beginning, or may have 

 become infected at any time from neighbouring mussels. The theory of transference 

 of the parasite from one mollusc (such as cockle) to another (the mussel) may be true, 

 but it is not proved by those experiments. It was not shown that the mussels were 

 free from parasites at the start, the numbers in the recorded experiments are too 

 small to yield definite conclusions, and the observations should clearly be repeated, 

 using hundreds of cockles and of mussels with well-devised control experiments. In 

 order to show the necessity for large numbers in this kind of work, it may be added 

 that, Mr. Andrew Scott having informed us of Dr. Jameson's observations at Piel, we 

 had some samples of these same mussels and cockles sent to the Liverpool Laboratory, 

 * 'Comptes Renrlus Soc. Biol. Paris,' December 19, 1903, lv., p. 1618. 



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