108 CHROMOSOMES DURING SYNGAMY IN PROTOZOA 



equatorial plate. The pole caps become the karyosomes of the daughter 

 nuclei. 



5. Division in which the karyosome becomes elongated and divided 

 within the nuclear membrane without development of spindle fibres 

 or chromosomes. This type of division is seen in the nuclei of small 

 organisms, and it is probable that it is actually similar to type 4, the 

 spindle fibres and chromosomes escaping detection owing to their 

 minuteness. 



In those cases in which a centrosome is not present, some observers 

 claim that its place is taken by an intranuclear centriole. 



BEHAVIOUR OF CHROMOSOMES DURING SYNGAMY. 



Reference has already been made to the nuclear changes which occur 

 during the development of the ovum and the spermatozoon, and it has 

 been pointed out that the chromosome number of the zygote nucleus is 

 not doubled as a result of syngamy owing to the fact that after meiosis 

 the nuclei of the uniting gametes contain half the normal number of 

 chromosomes. Several instances of similar reduction divisions of Pro- 

 tozoan nuclei, whereby the number of chromosomes is halved, have been 

 recorded. 



Muslow (1911) gives a clear account of a supposed reduction division 

 in Monocystis rostrata (Fig, 64). The nuclei of the two gregarines which 

 enter the gametocyst multiply by repeated mitotic divisions in which 

 eight chromosomes are present, as noted on p. 92. Eventually, after 

 nuclear division has ceased, a number of gametes are budded off from 

 each gregarine, and these unite in pairs and their nuclei fuse. During 

 the last nuclear division, whereby the gamete nuclei are formed, though 

 eight chromosomes appear on the equatorial plate, when the daughter 

 plates are formed, there is no splitting of the chromosomes, as has occurred 

 in previous divisions, but the eight chromosomes are separated into two 

 groups of four, which move towards the poles of the spindle. It was noted 

 also that the eight chromosomes composing the equatorial plate consisted 

 of four pairs of homologous chromosomes, the members of each pair 

 differing from those of other pairs, and that one of each pair entered each 

 daughter plate of chromosomes. This last division, which gives rise to 

 the nuclei of the gametes, is thus a true reduction division or meiosis, like 

 that which occurs in the production of gametes in higher animals. When 

 the gametes unite, the nucleus of the zygote, receiving four chromosomes 

 from each gamete nucleus, again has eight chromosomes or four pairs of 

 homologous chromosomes. 



In the case of Diplocystis schneideri, a gregarine of the cockroach, 



