HOW THE MAMMALS DEVELOPED 197 



being occupied and cleaned up by a sunfish. The 

 pebbles were lifted into the mouth of the fish and 

 driven out again with force. The water which 

 emerged with the stones seemed to wash away the 

 dirt, while the pebbles themselves became gradually 

 cleaned of the green plant life which ordinarily covers 

 them. After the process was completed each spot was 

 saucer-shaped and free from scum and mud. Over 

 each of these spots hovered the sunfish which made 

 it, and round and round the fish swam. The circles 

 thus traversed were so near each other that every 

 now and then the occupants of two adjoining nests 

 would meet on the border. The fish which was most 

 nearly on its own ground would at once attack the 

 other and drive him away. In a few days the other 

 partner in each family seemed to appear. Now two 

 fishes swam side by side over each nest, bringing the 

 lower edge of their bodies comparatively close to- 

 gether. In this position they moved around over the 

 pebbly bottom. The female was discharging her mul- 

 titudinous and very small eggs, so that they dropped 

 to the bottom of the nest. At the same time the male 

 was expelling what in fish is known as the milt. In 

 this milt are the sperm cells of the male, each consist- 

 ing of a rounded head and a very slender body. These 

 are attracted by the eggs. Pushing up against them, 

 the head of a sperm cell, consisting almost entirely 



