66 ILLINOIS BIOLOGICAL MONOGRAPHS [66 



temperature of 20°C (68°F), and reaches its climax with the water at a 

 temperature of 22.4°C (72.9°F), the place of observation being the north 

 shore of Oconomowoc lake, where the silversides breed abundantly over 

 the sand and gravel bottom. These facts are brought out in table 7. 



Table 7 



TEMPERATURE OF WATER OF OCONOMOWOC LAKE ASSOCIATED 



WITH BREEDING PHENOMENA OF LABIDESTHES SICCULUS 



Degrees Centrigrade 



1920 1921 1922 1923 Av. 



Vertical pairing first noted 18.3 17.8 17.7 18.2 18.0 



First spawning noted 20.3 20.0 20.0 19.7 20.0 

 Climax of spawning 23/2 22A 22^ 22_1 22.4 



After extrusion the egg settles slowly to the bottom. It is armed with an 

 "organ of fixation," consisting of a single gelatinous filament of a length 

 equal approximately to six times the diameter of the egg. This is called 

 by Hubbs an organ of flotation, but its primary function seems, rather, to 

 be that of fixation. It adheres to the first thing with which it comes in 

 contact, either vegetation or bottom material, and the egg is firmly at- 

 tached. Where spawning occurs in rivers, the secondary function of 

 flotation may well be of some importance in relation to distribution by 

 current, but as the species is so active and this activity carries it such long 

 distances, this slight distributional value must be regarded as subordinate 

 to the fixation function. As the egg itself is not at all viscous, it would 

 become buried in the debris and sand at the bottom, did it not become 

 fixed before reaching it. 



DEVELOPMENT 



Nothing whatever is known regarding the embryological development 

 of the species, and the present writer has not considered this phase of the 

 life of the species as within the scope of his problem. That development is 

 quite rapid is indicated by the fact that eggs hatched in eight days in the 

 writer's laboratory in water kept at approximately 25°C (77 F). This 

 temperature is two degrees above that of the lake from which the specimens 

 were taken. Observations on eggs located in the lake and observed daily 

 indicate that the hatching period is very close to 8 days (between 8 and 9) 

 with the water at 23°C. That temperature affects the rate of development 

 is very probable from what is known concerning other species, in which 

 the rate of development increases with an increase in the temperature 

 of the water; therefore one would expect a variation in the normal hatching 

 periods of the eggs in a lake depending upon the fortuitous chances of the 

 eggs becoming fixed in water of varying depths. For it must be remembered 

 that since the eggs are laid in shallow water, a considerable range of 

 temperature variation will be encountered, the range being greater the 



