PEARL PRODUCTION. 25 



Figs. 8 and 10, on Plate L, show examples of this from Mytilus edulis, where the 

 sections were stained with eosine and methyl blue, and in both ectoderm and pearl- 

 sac the cells and nuclei are of the same size and shape, and the nuclei are stained 

 red with eosine and the cytoplasm blue to the same extent, so as to have a precisely 

 similar appearance. In looking at a small part of the section under a high power, 

 one receives the impression that the two adjacent epithelia are folds of the same 

 layer (see Plate I., fig. 12). We show the same point in the case of the Ceylon 

 pearl in fig. 16. Here the section is stained with gentian violet and light green, and 

 in both ectoderm and pearl-sac the nuclei have taken up the violet, and the cytoplasm 

 the green, to a quite similar degree. 



2. The fact that pearls formed in different parts of the mantle have the character 

 of the layers of the shell formed by the ectoderm in their neighbourhood " horny " 

 pearls, resembling the periostracum, have been found at the mantle edge ; in the zone 

 above that, pearls have been found having the characters of the prismatic layer of 

 the shell ; and finally, the great majority of pearls, both in the mantle and in the 

 deeper tissues, show the structure of nacre, the layer produced by the greater part ol 

 the surface of the ectoderm on the mantle. It is difficult to account for these facts 

 if the epithelium of the pearl-sac has no genetic connection with the layer of 

 ectoderm lying outside it. The ordinary nacreous pearl is clearly produced in a 

 similar manner to the inner part of the shell. The nacre is formed from epithelium 

 on the outer surface of the mantle ; the pearl from epithelium lining a closed sac. 

 The most natural working hypothesis to hold until it is disproved, is that the 

 epithelium of the closed sac is derived in some manner from the outer surface of the 

 mantle. 



Against this view, however, there is the notable fact that most recent investigators* 

 have been unable to find any evidence of the pushing in of the ectoderm to form the 

 pearl-sac. We may feel fairly certain, then, that the majority of pearls are not 

 formed in actual pouches of ectoderm closed off from the mantle, as, if such structures 

 were formed in any numbers, we could scarcely fail to obtain some evidence of their 

 presence. 



* The one definite exception to this statement is the case of M. L. BOUTAN, who, in his paper in 1904 

 (" Les Perles fines : leur origine re"elle," ' Arch. Zool. Exper.,' 4 ser., tome ii., p. 47), describes and figures 

 the actual pouching in of the ectoderm around the Distomid parasite to form cyst pearls in the case of 

 Mytilus edulis. In this paper BOUTAN criticises adversely GIARD'S comments on our short note read at the 

 Southport Meeting of the British Association in 1903; but our intention in that paper certainly was to 

 express our belief in the ectodermal origin of the cyst pearls. When we stated " In all cases, whatever 

 its nucleus may be, the pearl,' like the nacre, is deposited by an epithelial layer," we intended to imply the 

 ectoderm ; and where, further on, we speak of " closed sacs," we meant to indicate that the ectodermal 

 pouches alluded to in the previous sentence have become closed off. Professor GIARD interpreted our 

 words correctly ; and although in the present report we have discussed both possible views, still we have 

 from the time of our first observations in Ceylon believed, as BOUTAN does, in the ectodermal origin of the 

 cyst or " fine " pearls, 



