62 



Histology. (Text fig. 9). 



Before autotomy. A longitudinal section through 

 the basi-ischium of a pereiopod in which autotomy has 

 not been effected displays the following structure (9J..). 



In the region of the fracture plane the exoskeleton 

 is discontinuous, the plane of the discontinuity being at 

 right angles to the longitudinal axis of the basi-ischium. 

 The break is not always easily detected, as the two parts 

 fit very closely together. 



On the inner side of the exoskeleton is the normal 

 layer of epidermis (cp.). At the plane of breaking the 

 epidermis turns inward both at the distal extremity of the 

 basipoditic region and also at the proximal end of the 

 ischiopodite. These ingrowths extend as far as the 

 central nerve and blood vessels, where the epidermal 

 ingrowth of the basipodite (i.) becomes continuous with 

 that of the ischiopodite (o.). In other words, across the 

 plane of fracture the epidermis underlying the exo- 

 skeleton is not directly continuous, but becomes turned 

 inward as far as the central nerve of the leg. 



Thus there is a double diaphragm stretching across 

 the leg in the fracture plane, and near the centre of this 

 double membrane there is a small opening which permits 

 of the passage of the nerve (».) and blood vessels from 

 one side to the other. The walls of this narrow opening 

 are composed of a cellular membrane, which connects 

 the proximal and the distal diaphragms. 



After autotomy. The ischial portion of the exo- 

 skeleton is broken away at the fracture plane, and the 

 underlying structures belonging to the ischium have also 

 been torn away. These include the epidermis of the 

 ischium and also the distal portion of the diaphragm. 

 Stretching across the broken end of the stump (Text 



