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 
