276 



THE EVOLUTION THEORY 



former students, Dr. Stuhlmann and Dr. Schwarz, the essential point 

 seems to be that the colossally large zooaperms, which show no activity 

 within the body of the male, leave it one at a time, so to speak, in 

 single file. In copulation they are pressed out singly, one after the 

 other, through a very fine tube, and then they enter, still singly, 

 through the reproductive aperture of the female into an equally fine 

 passage with spiral windings, through which they ultimately reach a 

 roomy pear-shaped receptacle, the 'receptaculum seminis' of the female. 

 There they lie in a long band composed of several hundreds, and 

 only now attain their full maturity by throwing off an outer cuticle — 

 moulting, so to speak. It is only when they get into a fluid medium 

 that they show the power of undulatory movement, feeble at first, 



but gradually more ener- 



bd^ 



getic and more violent. 

 And these movements 

 enable them to penetrate 

 like gimlets into the 

 calcareous egg-shell. In 

 the normal course it hap- 

 pens that when a mature 

 ovum is deposited from the 

 opening of the oviduct, 

 one of the giant zoo- 

 sperms at the same time, 

 or shortly afterwards, 

 leaves the ' receptaculum 

 seminis ' of the female by 

 way of the spiral passage, 

 and reaches the exterior 

 just behind the ovum. 



The actual process of penetration has not been observed as yet, but 



the zoosperm has been seen at a sKghtly later stage spirally coiled 



inside the ovum. 



In these Ostracods the sperms are often visible with the naked 



eye, and in some species they are twice the length of the animal; 



they are thus emphatically giant cells, which can develop a very 



considerable boring power. 



In respect to the various adaptations of the sperm-cells to the 



conditions of fertilization there is hardly any group more interesting 



than the water-fleas or Daphnids. 



It is amazing how greatly the size of the sperms varies among 



the Daphnids, and how it stands in inverse proportion to their 



Fig. 65. Copulation in a Daphnid (Lyncseid). 

 Emptying of the sperm (sp) into the brood-chamber 

 of the female (?). abd i, the abdomen of the male. 

 Magnified 100 times. 



