ANNUAL MIGRATIONS 127 



ductive organs of the many samples taken. At the 

 minimum phase all the tissues incorporated appear 

 to be in a state of rest. After recrudescence has 

 commenced, interstitial tissue begins to appear, 

 reaches a maximal development and then, as the 

 organs get markedly larger, again disappears almost 

 entirely, a few scattered cells remaining here and 

 there. The sex elements are now getting active and 

 the eggs in the ovary and the tubules in the testes 

 attaining their greatest size. Regression from the 

 maximum is at first extremely rapid but the rate 

 soon diminishes and when the organs are approach- 

 ing their minimal condition, interstitial tissue again 

 develops, gets quite abundant, subsequently to dis- 

 appear again as the gonads finally reach their winter 

 condition. Interstitial tissue is thus present and 

 at its maximum just when the organs are in the 

 phase found in birds at the height of their 

 migrations. 



Having, then, discovered a method of manipulat- 

 ing the annual rhythm of the reproductive organs 

 and having procured details of their histology 

 through the entire cycle, it became possible to liber- 

 ate birds at known stages of development and to 

 ascertain something of their behaviour. According 

 to our hypothesis it should be possible to retain 

 juncos at Edmonton, hundreds of miles north of 



