SPERMIOGENESIS OF THE PRIBILOF FUR SEAL 481 



a line. In this way the perforatorium is formed, a structure 

 less conspicuous in the fur seal than in many other mammalia. 



In the earlier part of this period the remnant of the sphere is 

 visible in the distal region of the cell as an irregular mass, slightly 

 denser than the surrounding cytoplasm. Here it undergoes a 

 gradual degeneration, eventually disappearing from view. 



Shortly after the fusion of the anterior centrosome with the 

 nuclear wall, which takes place near the end of the first period, the 

 posterior centrosome undergoes a division into two equal parts 

 (figs. 21 to 23). At about the same time there appears a rod- 

 like prolongation from the anterior centrosome, extending ob- 

 liquely downward and outward in the cytoplasm inclosed by the 

 caudal tube. This is the so-called 'batonnet,' or rodlet (figs. 

 20 to 25), a well marked structure in the fur seal. It increases 

 in length steadily and finally touches the wall of the caudal 

 tube (fig. 27). 



Following the division of the posterior centrosome its distal 

 half migrates along the axial filament to take up its definitive 

 position at the end of the connecting piece. As this marks the 

 beginning of the third period it will be described in that con- 

 nection. 



The vesicle described by Meves ('99) for the guinea-pig, and 

 by Duesberg ('08) for the rat, upon the tail filament, is also 

 present in the fur seal, but has been observed in only a few 

 cases. It appears to be of similar origin and relation to the 

 tail filament, but its history has not been followed in this 

 material. 



The third period: from the migration of the annulus along the tail 

 filament to the passage of the spermatozoids into the lumen of 

 the tubule: figures 30 to 82 and 34 to 35 



The progressive lengthening of the spermatozoid has now 

 shifted the main mass of the cytoplasm to the distal end, leaving 

 only a thin layer covering the caudal tube, the cell membrane 

 appearing as a delicate line. In many preparations this is seen 

 with difficulty, especially as the spermatids soon enter into rela- 



