40 THE RIDDLE OF MIGRATION 



do in midwinter. In tropical birds the difference 

 is slighter. In semitropical birds, such as the 

 canary, the winter testes are never as small as in 

 truly northern birds of similar size. 



Obviously, with such marked seasonal disparity 

 in actual size we must look also for changes in 

 internal structure. The typical spring avian testis, 

 cut across the middle and examined under the 

 microscope presents the picture shown in Fig. 7A. 

 The larger part of it is made up of coiled tubules 

 {seminiferous tubules) bound together on the out- 

 side by a sheath, {tunica alhuginea) and communi- 

 cating with the sexual opening at the vent by means 

 of a duct termed the vas deferens. In these tubules 

 are produced the male sex cells, the spermatozoa, 

 during the breeding season. At other times the 

 tubules are quiescent and inactive and much dimin- 

 ished in size, the distention during the breeding 

 season being largely due to the enormous multiplica- 

 tion of the contained cells. Within the testis the 

 tubules are separated from each other by tissue 

 which is not concerned directly with reproduction 

 and has no duct communicating with the outside. 

 It consists chiefly of connective tissue cells in which 

 run blood and lymphatic vessels. In many wild, 

 particularly migratory, species large glandular look- 

 ing cells are to be found scattered through this con- 



