Ill 



CHONDRIOSOMES 69 



filamentar. During cell division they congregate between the separating 

 daughter nuclei. Finally, when cell division is almost complete, they 

 occupy the bridge of cytoplasm that connects the two nearly separated 

 daughter cells. WTien this is broken through, the bundle of chondrio- 

 somes is also broken across, and thus a portion of the chondriosome mass 

 is left in each daughter cell. 



In the young primary spermatocyte they are again scattered through 

 the cytoplasm, but at the beginning of the growth period they concentrate 

 round the centrosome, forming thus a cap at the pole of the nucleus. 



They remain in this position throughout the growth period, but in 

 the later prophase again become scattered through the cell and form 

 during mitosis a mantle round the spindle figure. Their distribution 

 between the two secondary spermatocytes is thus insured. 



In the second division they behave in the same way, and thus each 

 spermatid receives a share of the chondriosomes present in the primary 

 spermatocyte. 



In the spermatid they fuse together into a compact mass which may 

 be as large as, or larger than, the nucleus. This is the supplementary 

 nucleus, or " Nebenkern," of older cytologists (though this term has also 

 been applied to structures of different natures). 



Next, the " Nebenkern " divides into two, and elongates with the 

 lengthening tail of the spermatid, the two portions clasping between 

 them the axial filament of the tail. As the spermatid continues to 

 elongate, its parts become more and more attenuated, the " Nebenkern " 

 keeping pace with this elongation, and undergoing various minor changes. 

 In the adult spermatozoon it forms a sheath for nearly the whole length 

 of the tail, only a very short stretch of the latter projecting from the end 

 of the sheath. 



In the spermatids of many animals the chondriosomes are scattered 

 instead of being concentrated into a " Nebenkern." In the spermatozoon 

 they are found in a variety of positions, but apparently are never absent 

 from the ripe spermatozoon.^ This fact has led certain cytologists to 

 assign a very important function to the chondriosomes (Chapter VI.). 



We are now in a position to studj^, very briefly indeed, the general 

 development of the spermatozoon from the spermatid. The development 

 of the mammalian spermatozoon as worked out for the guinea-pig by 

 Meves (1899) and Duesberg (191 1 a) will serve as a type (Fig. 30). Very 

 briefly, the course of development is as follows : After the second 

 meiotic division the spermatid consists of a cell containing (i) the nucleus ; 

 (2) chondriosomes (here in the form of scattered granules) ; (3) two centro- 

 somes ; (4) the idiosome. The last-mentioned body corresponds more 

 or less closely to the attraction sphere of other cehs. In the spermatid 



^ An exception has been described in the case of Ptii pains (p. 197). 



