258 Proceedings of the Eoyal Society of Edinburgh. [Sess. 
regard the second limb-segment as a rigid cylinder, forces applied at 
the points of insertion of the extensor muscles, and in the line of their 
action, must cause great weakening of the cylinder at the region of the 
breaking-plane. So, if the distal part comes into contact with a resistant 
point, breaking at that level will be very easily produced, as occurs in 
Cancer. In Portunus, where the leg may he shot off without meeting a 
resistant point, the opposing action of the long and short extensors may 
be so great as actually to cause splitting at the breaking-plane. 
Fredericq pointed out that autotomy could not occur if the limb were 
forcibly extended. This position is the only one in which the short 
extensor cannot act, and Fredericq did not appreciate the fact that its 
powerlessness was the reason why autotomy could not occur. 
Recognition of the weakening at the breaking-plane (caused by the 
Fig. 29. — Explanation in text. 
opposing action of the extensors at the point of junction in the ischial 
ring of the second segment), which I have demonstrated, shows that 
Demoor’s torsion theory is unnecessary. Fredericq and Demoor have 
criticised one another to no purpose, for they have worked on different 
animals without making allowance for changed conditions. They both 
state emphatically that they have not examined one another’s types. 
To prove my hypothesis that weakening at the breaking-plane, as the 
result of opposing muscular contraction, is the sine qua non of autotomy, 
I inserted a knife blade firmly between the two parts of the lower ring, 
and found that the limb snapped oft at the breaking-plane on the slightest 
force being applied laterally to the distal part (“ partie caduque ”). When 
this is not done, no amount of twisting and pulling can accomplish division 
at the seat of election. 
Thus, summarising, it can he said that autotomy is a unisegmental 
reflex in the Brachyura. It is the result of co-ordinate action of the 
basal muscles of the limb attached to the second segment, and follows 
weakening of the breaking-plane region. It is most probable that its 
chief end is to prevent haemorrhage, for division of the leg at the breaking- 
plane allows the venous valve at that level to close. An external point of 
