100 ILLINOIS BIOLOGICAL MONOGRAPHS [100 



The following data apply to the gonads of the male : 



Average weight of 45 right testes 7.5 grams 



Average weight of 45 left testes 6.5 grams 



Total weight of gonads 14.0 grams 



In every case the right testis exceeded the left in weight. 



In order to ascertain the effect of temperature upon the fish, twenty-five 

 ciscos were captured by means of a gill net and the ten least injured 

 were brought in alive to the vivarium, where they were placed in a large 

 concrete tank abundantly supplied with a constant supply of fresh running 

 water. The fish lived well throughout the winter, with only an occasional 

 mortality due to the fungus Saprolegnia. The water was kept at a tem- 

 perature of 4.5°C during a period of four months, covering the breeding 

 season. In spite of the fact that fifteen of the confined fish were females, 

 all heavy with eggs, not a single egg was laid during this time. In a second 

 tank, exactly similar to the first, and with the same water supply, but 

 cooled by means of ice to a temperature of 3.5°C, females from the first 

 tank spawned within ten minutes after transfer. 



A second experiment consisted in transferring two females into the 

 second tank while the water was at 4.5°C. After two hours in this tank, 

 a large piece of ice was added and a careful record of the temperature kept. 

 The first female spawned with the temperature at 3.6°C, the second at 

 3.4°C. 



From the fact that the observed temperatures at which the fish spawn 

 in nature coincide exactly with the observed experimental controls in the 

 laboratory, namely 3.3° to 3.6°C, it must be concluded that these figures 

 represent the critical breeding temperature of Leucichthys artedi. It must 

 be further concluded that the temperature is the causal factor for egg 

 deposition and that this critical temperature is needed before the eggs 

 will be laid. This conclusion is inescapable as repeated oxygen and hydro- 

 gen-ion concentration determinations showed the conditions in the water 

 to be almost constant for these factors, and the carbon dioxide variations 

 were so very slight and in both directions that they must be considered 

 as having no significance. . 



Nothing is known concerning the embryology of the species, and here 

 again the writer has considered the subject as outside of his problem. 

 However, it is well to point out that this egg is an ideal one for laboratory 

 use and experimental purposes: it is easily obtainable, available for 

 winter work, development is sufficiently slow for convenient study, and 

 the egg is of fair size and remarkable clearness, having only a very few oil 

 globules to obscure the cleavage. The writer has kept them in regulation 

 egg-hatching glass jars until the embryo was well developed, when the 

 set was accidentally destroyed by the breaking of the jar. These eggs were 



