SPERMATOGENESIS OF THE DRAGON-FLY. 283 



head-spine which after increasing from .01 mm. to .45 mm. in 

 length diminished with the ripening of the spermatozoa until 

 it was only .ooyS-.oog mm. long. Fig. 74 is a drawing from a 

 smear preparation showing the shape of the nucleus and the 

 elongated head spine in the spermatid. The nucleus which at 

 first is round, elongates and apparently enters the clear zone 

 comprising the head-spine. The cytoplasm becomes a thin 

 sheath around the nucleus which is extremely long and narrow. 

 There is never a distinct middle piece, but the knob around the 

 centrosome may be homologous to the middle piece found in 

 other spermatozoa. In the ripe spermatozoa, this does not 

 appear separated from the nucleus and the whole head stains 

 as if composed of solid chromatin. The chromatin, however, is 

 collected around the outer side of the nucleus, as cross-sections 

 (Figs. 75 and 76) show a denser staining band of chromatin 

 just beneath the nuclear membrane. The adult spermatozoa 

 are motile and swim along with a twisting spiral motion. Figs. 

 77, 78, and 79 are drawings from aceto-carmine preparations in 

 which the spiral twist of the adult spermatozoon is indicated. 



There are apparently no external differences among the sperma- 

 tozoa. In many spermatids drawn to scale no variation in size 

 of head could be detected. Zeleny and Faust ('15) have found 

 a dimorphism in the spermatozoa of Mschna canadensis, the 

 ratio of the two i.oo : 1.03. It is possible that using their 

 method a dimorphic curve could be plotted for Sympetrum. 

 However, the sperm on account of their method of locomotion 

 would be fixed in varying twisted postures. When these are 

 stained, their twisted condition is noticeable in only a few cases; 

 most of the stained specimens appear as rods. This fact alone 

 might cause such a discrepancy in size among the adult sperma- 

 tozoa as to be mistaken for dimorphism even should the latter 

 not occur. In Sympetrum from the mode of formation, one 

 half the spermatozoa has more chromatin than the other half 

 and this extra chromatin brought in by the sex-chromosome 

 probably causes a physiological difference even if there is no 

 visible dimorphism. 



