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. 
Fic. 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 ; s¢ XJTV; last thoracic sternum ; td, aperture of vas deferens, 
