136 



THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



of what occurs in other animals, however, leaves no doubt 

 that an actual mixture of the male and female ele- 

 ments takes place and constitutes the essential part of 

 the process of impregnation. 



Ova to which spermatozoa have had no access, 

 give rise to no progeny ; but, in the impregnated ovum, 

 the young crayfish takes its origin in a manner to be 

 described below, when the question of development is 

 dealt with. 



Fig. 35.— Astacus fluviatilis. — The last thoracic sternum, seen from 

 behind, with the proximal ends of the appendages, A, in the male, 

 B, in the female, ( x 3). am, articular membrane ; exp, coxopo- 

 dite ; st XIV, last thoracic sternum ; vd, aperture of vas deferens. 



