EXPERIMENTAL METAPLASIA 357 



slightly apart, the tentacles are expanded, and it responds rap- 

 idly to any stimulus by closing the shell ; when held up in the air 

 the water which drains away is clear and contains no slime. An 

 unhealthy specimen lies with the valves of the shell wide open, 

 there is little or no response to stimuli, and the valves only close 

 under pressure. The tentacles are retracted, and the gonads, 

 gills, and tissues generally look flabby and unhealthy. The water 

 which flows out between the valves is slimy and viscid, and this 

 is generally the first sign of deterioration. 



When making an implantation of the ovary, a healthy speci- 

 men was chosen in which the gonad was of a full orange or ver- 

 milion color, and obviously distended with ova. The valves 

 of the shell were wedged apart with a cork, and the interior wefl 

 washed with a brisk stream of sterile sea water from a wash bottle 

 that had previously been steamed for some time in a 'Koch.' 

 The adductor muscle was then severed with a scalpel, and one 

 valve of the shell turned back. The extremity of the ovary was 

 cut off, and while held b}^ forceps, was very thoroughly washed 

 with a stream of sterile sea water, then it was placed in a steril- 

 ised Petri dish containing a little sterile sea water, and kept care- 

 fully covered. 



All instruments were sterilised by boiling in dilute caustic soda 

 solution, and were washed in sterile sea water to remove any trace 

 of the caustic soda immediately before use. They were always 

 re-sterilised between each experiment. When not in use it was 

 found more convenient to keep all steel instruments in lime water 

 instead of drying them, as they are particularly liable to rust 

 after being exposed to the action of sea water. 



The transplanting needles, made of a platinum and iridium 

 alloy, resemble large hypodermic needles. Two sizes were used, 

 the larger for experiments on P. maximus, measured about 6 

 cm. in length, and 1 mm. in diameter, the smaller for experiments 

 on P. opercularis was of the same length, but about half that 

 diameter. Into the hollow needle a somewhat longer stilet fits 

 closely, and works like a piston. Any material taken up in the 

 point of the needle is sucked in by drawing the stilet back, and 

 again ejected by pushing it forward. Small portions of the ovary 



