spermatogenesis. xxi 



granules become aggregated into rod-like chromosomes 

 of equal length and of constant number in each species ; 

 these are formed at the equator of the spindle, and 

 split by longitudinal fission, so that their number 

 becomes doubled. One star of chromosomes is drawn 

 to the centrosome at each pole of the spindle, and thus 

 two daughter-nuclei, which for convenience I will term 

 oocytes, are formed, of unequal size ; the smaller one 

 being extruded as the first polar bod3^ In the 

 parthenogenetic G:gg this completes the division, but 

 in the sexual Ggg a second nuclear division follows 

 immediately on the first without a resting stage ; the 

 oocyte divides into two oozoa, and one oozoon, contain- 

 ing half the chromosomes, is extruded as the second 

 polar body. The oocyte which forms the first polar 

 body is observed to split into two oozoa after extrusion. 



In this way three of the oozoa which have arisen 

 from the division of the primitive germ-cell have 

 become polar bodies, while the remaining oozoon, 

 containing one half the number of chromosomes which 

 the primitive germ-cell contained, is left in the nucleus, 

 functional or capable of development. 



Within the spermatic tubes of the male, corresponding 

 changes take place in the primitive sperm-cell. Its 

 chromosomes are doubled and it divides into two sper- 

 matocytes (spermatogens), each of which again divides 

 without a resting stage into two spermatides (spermato- 

 blasts). The four spermatides undergo various changes 

 during which they become elongated, the nucleus, 

 containing the chromatin elements, becomes the head 

 of a spermatozoon and ends in a motile barb. The 

 protoplasm of the cell-body is drawn out into a sheath 

 through which the filament passes, and is continued 

 beyond as the vibratile tail. 



