22 THE OYSTER. 



squatting with his legs crossed under him, and his 

 elbows on his knees : they are formed of pearl pre- 

 cisely like that which lines the rest of the shell, a little 

 raised above its surface and outlined in faint relief, 

 but they are part of the shell, with no break nor joipt. 

 In the process of manufacturing them, the shell of 

 the living animal is wedged open, and thin images, 

 punched out of a sheet of bell-metal, are inserted. 

 The animal is then returned to the water, and is left 

 there until enough new shell has been formed to 

 ^ cover them with a varnish of pearl thick enough to 

 fasten them, and to hide the metal, while permitting 

 the raised outline to be seen. 



Several years ago it occurred to me that a series of 

 microscopic specimens of stages in the growth of the 

 shell might be obtained in the same way, and that, by 

 studying them, the whole history of the process might 

 be traced. ^One of my students, at my suggestion, put 

 into the shells of a number of oysters thin glass circles, 

 such as are used to cover microscopic specimens. The 

 oysters were then returned to the water, and were left 

 undisturbed until new shell began to be formed on the 

 glasses. These were then taken out and studied under 

 the microscope. 



At the end of twenty -four hours the glass was found 

 to be covered by a transparent, faintly brown film of 

 thin gummy deposit, which exhibited no evidences of 

 structure, and contained no visible particles of lime, 

 although it effervesced when treated with acids, thus 

 showing that it contained particles too small to be 

 visible with a microscope. The gummy film is poured 

 out from the wall of the mantle, and in forty-eight 



