VIII] THE ORIGIN OF PEARLS 111 



leave the Ceylon pearl oyster for a while and turn to 

 the most recent scientific work on the fresh-water 

 mussel in Germany. 



Rubbel, working at Marburg, has once more 

 sho^vn how often the older scientists came near 

 to the solution of problems not finally solved till 

 to-day. 



According to the investigations of this worker 

 the fresh- water mussels {Margaritana margaritifera) 

 examined by him contained few parasites that could 

 give rise to pearls. Parasites were, moreover, not 

 found to be the cause of pearl formation in this 

 shellfish. The nuclei of the pearls consisted of par- 

 ticles of a yellow-brown substance, that was strongly 

 refractive. This substance could be found scattered 

 in the connective tissue of the mantle and also in the 

 shell-secreting epithelium of the mantle. Further- 

 more it seemed to be related to the substance of the 

 outer layer of the shell, i.e. the periostracum. In 

 short, we are reminded of the views of von Hessling, 

 put forward in 1859. 



In order to understand fully the mechanism of 

 pearl formation, Rubbel first made experiments to 

 determine what the outer shell-secreting epithelium 

 of the mantle could do in the way of repairing broken 

 shells or regenerating pieces of shell that were re- 

 moved. He discovered that the nacre-secreting 

 epithelium, normally secreting the mother-of-pearl 



