SEA-FISHERIES LABORATORY. 349 



distal portion of the limb comes into contact with the 

 carapace or with some other fixed object, when the limb 

 breaks at the fracture plane. That this upward move- 

 ment, or extension, of the basi-ischium is necessary for 

 autotomy may be proved by cutting the extensor muscle 

 for muscles), and then injuring the limb. ~No self- 

 amputation will then take place. If the flexor muscle 

 be cut, and the extensor remain uninjured, autotomy will 

 proceed. 



5. The distal portion of the limb must come into contact 



with some point of resistance. 



This condition has been emphasised above. As soon 

 as that part of the limb on the distal side of the fracture 

 plane comes into contact with some point of resistance 

 (e.g., the carapace) the upward movement of this portion 

 is stopped. The proximal portion of the basi-ischium, 

 however, still continues to move upwards under the 

 influence of the extensor muscle. Thus there are two 

 forces acting on the fused basi-ischium — a force at the 

 proximal end tending to move the segment upward, and 

 a force at the distal extremity preventing this upward 

 movement. A great strain is produced on the basi- 

 ischium and it snaps at its weakest point, which is the 

 fracture plane. 



6. The stimulation to produce autotomy must be applied 



between the fracture plane and the distal end of the 

 propodite. 



The nerve does not pass into the dactylopodite, so 

 that if the latter segment be wounded the nerve will not 

 be stimulated. It is equally futile if the limb be 

 damaged on the proximal side of the fracture plane. 



