MEIOSIS 111 



each six chromosomes. Finally, male and female gametocytes which give 

 rise to male and female gametes are formed. The nucleus of the male 

 or microgametocyte multiplies by repeated divisions in which the series 

 of six chromosomes are present (Fig. 66, A to D). They are filamentous 

 except when arranged as the equatorial plate, when they are contracted 

 and more or less spherical, though maintaining the same relations as 

 regards size. At the equator of the spindle the chromosomes divide by 

 constriction, and the two groups of six daughter chromosomes separate 

 and. become filamentous again. By repeated divisions of this kind, in 

 which the daughter asters divide before actual nuclei are formed, very 

 complicated poly-aster figures are produced. Eventually, as in the schizont, 

 nuclei which lie on the surface are constituted, and from them the micro- 

 gametes are formed. The latter are elongate bodies provided with two 

 flagella at the anterior end (Fig. 376). Meanwhile, certain merozoites of the 

 female line have become female- or macro-gametocytes. A complicated 

 series of changes takes place in the nucleus. The nucleolus or karyosome 

 is thrown out, the nuclear membrane disappears, and a series of six long 

 chromosomes appears (Fig. 66, E). Finally, a fertilization spindle is formed, 

 on w^hich the chromatin of the female nucleus is arranged in the form of 

 granules (see p. 873). The chromatin of the male nucleus, derived from 

 the microgamete, now enters the spindle, which retracts to form the zygote 

 nucleus (synkarion). This nucleus now proceeds to division by mitosis, 

 and the chromosomes are reconstituted (Fig. 66, F to K). It is found 

 that there are twelve of these — a series of six pairs, the two constituting 

 each pair being equal in size. Undoubtedly one chromosome of each 

 pair is derived from the microgamete nucleus and one from the macro- 

 gamete nucleus. The twelve chromosomes now pass to the equator of 

 the spindle and become globular in form, and the two constituents of 

 each pair now unite, giving a stage in which there are only six double 

 chromosomes (Fig. 66, G). The union, however, is not permanent, for 

 separation takes place, and one chromosome of each pair passes to one 

 pole of the spindle, while the other goes to the opposite pole (Fig. 66, H). 

 In this process there has been no division of the chromosomes, so that 

 in each daughter group there are only six chromosomes, whereas in the 

 zygote nucleus (synkarion) there were twelve. The first division of the 

 synkarion is thus a true reduction division, whereby the original number 

 of six is regained. It will thus be seen that in every stage of development 

 of this parasite the nuclei have six chromosomes, except in the synkarion 

 formed by union of the male and female nuclei, in which there are twelve. 

 The daughter groups of six chromosomes resulting from the division of 

 the synkarion now proceed to division again, but, as in the case of the 

 nuclear multiplication in the schizont and microgametocyte, at each 



