ORIGINE REELLE DES PERLES FINES. 57 



les intéressantes expériences qu'il a faites sur l'infection artificielle des 

 Moules et développe diverses considérations qui ne rentrent pas dans 

 le cadre de nos recherches. 



Lyster Jameson est donc partisan de la distinction entre les perles 

 de nacre et les vraies perles fines. Comment comprend-il et explique- 

 t-il l'origine et le développement de la perle fine? 



Pour éviter toute confusion, je citerais, in extenso, les passages 

 qu'il me paraît nécessaire de reproduire en souvenir de l'assertion 

 fantaisiste de M. le professeur Giard signalée dans l'introduction : 



« Origin and Developement of the Pearl * » 



« The Trematode enters Mytilus edulis as a tailless Cercaria, and 

 at first may often be found between the mantle and shell. It is 

 probable that it reaches this position by boring throug the mantle, 

 but I bave not yet been able to find one in the act of doing so. The 

 larvœ creep about on the inner surface of the shell, and, after a 

 while, again enter the connective tissue of the mantle, where they 

 come to rest, assuming a spherical form. They seem to avoid the 

 more muscular parts of the mantle — no doubt because the absence of 

 a definite boring apparatus makes it difficult for them to pass 

 through the latter. When embedded in the tissues they are visible 

 to the naked eye as little yellowish spots, about 1/2 mm. in dia- 

 meter. 



« At first the worm only occupies a space lined by connective tissue 

 fibrils (PI. XIV, fig. 2), but soon the tissues of the host give rise to an 

 epithelial layer, which Unes the space and ultimately becomes the 

 pearl-sac (PI. XV, fig. 5, s.) 



« This epitheliian"^ appears to arise quite independently of the 

 outer epidermis, and is no doubt due to a spécifie stimulation on 

 the part of the parasite as other parasites e. g. Sporocysts, Ces- 

 tode larvœ, etc., are not surrounded by such a sac. 



« At first a few cells appear (PI. XIV, figs. 2, 3, pr.) which proli- 



' Lyster Jameson, toc. ct7., pp. i48 et i49- 



•* Ces passages ne sont pas soulignes dans le texte original. 



