The Chromosome Cycle in Coccidia and Gregarines. 87 



passes through a long series of stages closely similar to those which occur in 

 the young sexual forms. We have not heen able to study the nuclear 

 divisions of the schizont in such great detail as we have those of the sexual 

 parasites : but it is evident from our results, taken together with those of 

 Leger and Duboscq (1908), who have previously described these stages, that 

 the number of chromosomes is six throughout the entire asexual cycle. The 

 first division of the schizont nucleus is by multiple mitosis, the later 

 ■divisions by ordinary mitosis into two — as in the sexual forms. The typical 

 size-relations of the six chromosomes are probably recognisable throughout. 

 (We have not been able to study every division, but those which we have 

 all appear quite typical.) 



From the foregoing description, it will be clear that in A. eberthi 

 the chromosomes are six in number at every nuclear division in the life- 

 history with one exception. This is the division of the zygote nucleus 

 immediately succeeding fertilization. There are here 12 chromosomes, which 

 become halved to six in the course of this division. Eeduction thus occurs 

 immediately after fertilization — not during gametogenesis. The six chromo- 

 somes must be regarded as representing the haploid number — the diploid 

 number (12) being present in the zygote nucleus only, while its division is 

 a reduction division. Since the haploid number (6) occurs in both the 

 sexual and the asexual cycle, there is thus no differentiation in respect of 

 chromosome number in the two generations. 



2. The Chromosomes of Diplocystis schneideri. 



The life-history of D. schneideri is comparatively simple. It is passed in 

 a single host — a cockroach. We have studied it chiefly in Periplaneta 

 americana, but have found it also in Stylopyga orientalis. At a very early 

 stage in their development the parasites unite in pairs in the gut wall 

 of their host, and then fall into the body cavity. After a considerable 

 growth period in the associated condition, they form gametes. The nucleus 

 of each member of the pair gives rise, by repeated mitoses, to a great 

 number of very small nuclei — each of which finally becomes the nucleus of 

 a gamete. The partition separating the two individuals (gamonts) has by 

 this time disappeared ; and the gametes, when fully formed, fuse in pairs. 

 The nucleus of the zygote (sporoblast) gives rise to eight daughter nuclei 

 which become the nuclei of the eight sporozoites formed within each spore. 

 The sporozoites develop within another cockroach into body-cavity parasites 

 once more, and the life-cycle is thus completed. 



Although the mitotic figures at certain stages are very small, the behaviour 



