90 



Messrs. C. Dobell and A. P. Jameson. 



in the spore, six chromosomes are formed by the segmentation of the spireme 

 thread. These separate into two homologous groups of three each, which 

 clearly represent the two sets of chromosomes derived from the two gamete 

 nuclei which united to form the nucleus of the zygote. This first division in 

 the spore — immediately following fertilization — must, therefore, be regarded 

 as a reduction division. 



The haploid number of chromosomes in B. schneideri is therefore three, and 

 occurs in all the nuclear divisions of the gamont, and in all the spore 

 divisions except the first. The diploid number is six, and is found at only 

 one division in the whole life-cycle — the first sporal division, immediately 

 following fertilization. Eeduction thus occurs directly after fertilization, and 

 not during gametogenesis. 



Conclusions. 



Although it is not yet possible to make any definite statement concerning 

 the chromosome cycle of the Coccidia and Gregarines in general from the two 

 individual instances which we have investigated, nevertheless we believe that 

 certain justifiable conclusions can be drawn from the results of our work. 

 We will briefly indicate what these are. 



Both in Aggregata and in Biplocystis the chromosomes are so remarkably 

 constant throughout the entire life-cycle that we believe this must be the 

 case in other Coccidia and Gregarines also. The apparent exceptions, in 

 which " amitotic " and " chromidial " nuclear divisions have been described, 

 are probably in every instance referable to abnormal or degenerate conditions 

 in the organisms studied, or to faulty cytological technique. We now know 

 this to be true at least of Aggregata and Diplocystis and other Sporozoa which 

 we have ourselves studied. 



So far as we are aware, the chromosomes have not previously been exactly 

 counted in any coccidian. " Eeduction " has, however, been described in a 

 number of forms (cf. Schaudinn, 1900; Siedlecki, 1898; etc.); but it is 

 evident that this " reduction " has nothing whatever to do with reduction 

 properly so called, i.e. with halving of the chromosome number. The 

 " reduction " and " epuration nucleaire " described during gametogenesis are 

 merely stages in the disintegration and elimination of the karyosome, and 

 have nothing whatever to do with the formation of the gamete nuclei. These 

 " reductions " have probably been so termed because they occur during 

 gametogenesis ; but in Aggregata at least — and probably in other coccidia — 

 the chromosomes are present in the reduced number throughout this process. 



In Cyclospora two " reduction-divisions " have been described (Schaudinn, 

 1902) in the maturation of the macrogamete. No chromosomes were observed, 



