12 University of Michignri 



phenus appendix, this is the period when sex is being established 

 in the larvae (see Okkelberg, 1921), and during which pro- 

 found changes are taking place in the gonads. Since the germ 

 <.q\\ history in Ichthyomyzon iinicolor has not been studied, only 

 the suggestion can be offered of correlation between the appar- 

 ent low vitality of this period and the changes connected with 

 the establishment of sex. 



Another explanation of the sudden drop in the curve 

 between the first and second years may be that the smaller 

 larvae are more subject than the older ones to destruction by 

 fish, for which they undoubtedly serve as food. 



There is a gradual decrease in the number of larvae during 

 the third and fourth years. After this there is no decrease 

 in the series from Thunder Bay River and the curve even 

 indicates an increase during the sixth year. In the tributary 

 to the river, however, the decrease is quite gradual up to the 

 last year. This discrepancy in the two curves may be corre- 

 lated with a passive migration of the larvae down stream dur- 

 ing heavy rains and spring thaws. Such transportation of 

 larvae down stream has been suggested for Bntosphenus appen- 

 dix (Okkelberg, 1921). The larvae which during their early 

 life period were found in the smaller streams might well have 

 been carried into the river by the time of their fifth and sixth 

 years, and thus there should be an increase of larger larvae 

 in the collection from the river. 



In the above discussion it has been assumed that spawning 

 took place under identical conditions each year. It is conceiv- 

 able that the crop of larvae of one year might be greatly 

 reduced during or directly after the spawning season, through 

 the agency of floods or other circumstances appearing at a 

 time when the embryos or young larvae are least able to sur- 



