DAVIS: SPERMATOGENESIS. 147 



25. The monosome does not divide in the first division but divides 

 longitudinally and probably equationally in the second. 



26. The spermatids are dimorphic, one-half containing a mono- 

 some while the other half lack this element. 



27. The monosome remains compact for some time within the 

 nucleus of the spermatids, but later breaks up into fine granules in the 

 same manner as the autosomes. 



28. In Dissosteira Carolina the Nebenkern is not derived directly 

 from the remains of the spindle fibers but is probably formed chiefly 

 from the mitochondrion. 



29. In Dissosteira Carolina the axial filament is from the first con- 

 nected with a distinct centrosome, which is applied to the exterior 

 of the nuclear membrane. As the spermatid elongates the Nebenkern 

 becomes converted into an envelope surrounding the axial filament. 



30. In Dissosteira Carolina the head of the mature spermatozoon 

 is formed entirely from the nucleus ; the centrosome forms the greater 

 part of the small middle-piece ; while the tail is composed of a central 

 fiber, derived from the axial filament and Nebenkern, surrounded by 

 a cytoplasmic envelope. 



31. In Steiroxys trilineata the axial filament is apparently not at 

 first connected with a centrosome, which appears in the usual position 

 only when the axial filament is well developed. The axial filament is 

 from the first surrounded by an envelope of doubtful origin denser 

 than the surrounding cytoplasm. The Nebenkern migrates around 

 the nucleus and becomes applied to its anterior end. 



32. In Steiroxys trilineata the head of the mature spermatozoon is 

 formed from the nucleus and the Nebenkern, the latter developing 

 into the acrosome; the middle piece is formed chiefly from the cen- 

 trosome, which divides into four parts ; while the tail is composed of 

 the central fiber, derived in part from the axial filament, surrounded 

 by a cytoplasmic envelope. 



33. In no case is there evidence that the monosome is extruded from 

 the nucleus during metamorphosis or that the spermatids degenerate, 

 except in rare instances. Therefore the mature spermatozoa must be 

 dimorphic with respect to their chromatin content, although there are 

 no visible differences either in form or volume between the two types. 



34. Throughout the entire history of the germ cells there is strong 

 evidence for the individuality of the chromosomes. 



