GAMETOGENESIS 463 



volving a reduction as well as an ordinary equation division, is limited 

 to the maturation mitoses occurring in active testes and ovaries^ The 

 appended scheme will serve to show graphically the steps in spermato- 

 genesis and oogenesis, and the correspondences and differences between 

 the two. 



We may begin the detailed description with the last generation of 

 spermatogonia, the secondary spermatogonia. In the early resting stage 

 the nucleus is poly-vesicular, each vesicle representing one or several 

 chromosomes (Plate A, Fig. 1). Subsequently the separate vesicles fuse 

 to form a greatly lobulated nucleus. In Fig. 2 one vesicle is shown still 

 separated ; this probably represents the accessory or sex chromosome ; sub- 

 sequently it also fuses with the main nucleus and remains generally 

 indistinguishable from the other nuclear elements until early stages 

 of the succeeding generation of cells, the primary spermatocytes (Fig. 

 8). Figs. 3, 4, 5, 6 and 7 represent successive stages in the karyo- 

 kinetic division of the secondary spermatogonia. In the telophase of 

 the spermatogonial division and the earliest prophase of the primary 

 spermatocytes, as well as subsequently, the sex chromosome (allosome; 

 heterochromosome) does not pass through the diffuse and spireme phases 

 of the ordinary chromosomes (autosomes; euchromosomes) but remains 

 chromatic and relatively compact. Subsequently it condenses consid- 

 erably more (Fig. 8) and thereafter persists until late spermatid stages 

 as a compact, deeply chromatic body, generally of oval or bilobed form, 

 and usually close to or on the nuclear wall (Figs. 10 ; plate B, 14 and 20 ; 

 and plate C, 27). Following the early growth stages, when the nucleus 

 is in the resting or diffusive chromatic stage (Fig. 8), a series of 

 changes occur known as synapsis. The most characteristic phase is the 

 one in which the spireme becomes closely aggregated and polarized 

 (synizesis) and subsequently segmented, the segments becoming looped 

 and attached by their open ends to the nuclear wall over a constricted 

 area, generally in the vicinity of the sex chromosome. The loops now 

 free one attached end and unite in pairs by their free ends forming 

 taller loops, oriented similarly to the smaller loops (bouquet figure, 

 synaptenic nucleus). The number of loops is now half the original 

 number, which represented the full number of chromosomes. This 

 chromosome pairing is the essence of synapsis. An end to end union 

 as here described is termed telosynapsis (metasyndesis) ; side by side 

 union, described in certain forms, parasynapsis (parasyndesis). The 

 important point is the reduction to half (plus one, the accessory) the 

 original number (diploid) of chromosomes; the reduced number is 



