The Chromosome Cycle in Coccidia and Gregarines. 91 



however, and it seems to us probable that the phenomena described were 

 misinterpeted. No " polar nuclei " like those described in Gyclospora are 

 formed by Aggregata ; and from the study of the chromosomes, it is clear that 

 the occurrence of reduction-divisions at this stage is improbable, as the 

 haploid number of chromosomes is already present. 



In Gregarines the chromosomes have been more fully studied than in 

 Coccidia. Their number has been counted in several forms, and it is significant 

 that in some of these an odd number has been found. In addition to 

 D. schneideri with three chromosomes, we may note Echinomera hispida with 

 five (Schellack, 1907), Nina gracilis with five (Leger and Duboscq, 1909), and 

 Gregarina ovata with three (Schellack, 1912). It is somewhat surprising that 

 what is probably the correct explanation of this uneven number — namely, that 

 it is the haploid number — has not previously been given. It has been 

 suggested (Schellack, 1907 ; Leger and Duboscq, 1909) that the odd number 

 is due to the presence of an " accessory " or " unpaired " chromosome — it being 

 assumed that the odd number is the diploid number, and that reduction 

 occurs in gametogenesis. We can find no good evidence to prove that a true 

 " accessory " chromosome is really present in any of the organisms in which 

 it has been described. The authors admit that the supposed " accessory " is 

 characterized chiefly by its size — which is no criterion — though in G. ovata it is 

 hardly, if at all, distinguishable from the other chromosomes (Schellack, 

 1912). It is also stated that this " accessory " or " axial " chromosome helps 

 to form the karyosome of the resting nucleus ; though in G. ovata a karyo- 

 some is not formed. We believe that the " axial " chromosome possesses no 

 special significance, the not infrequent presence in Gregarines of an odd 

 number of chromosomes being explained by the fact that it represents, as in 

 D. schneideri, the reduced number. 



There has already been a good deal of speculation concerning " reduction " 

 in the Gregarines, though it rests upon a very small basis of fact. In G. ovata, 

 Paelder (1904) and Schnitzler (1905) have described the formation of a 

 " polar body " by an unequal division of the nucleus during the " maturation " 

 of the gamete. L^ger and Duboscq (1909) have described a similar 

 " reduction " in G. munieri, though two " reduction-nuclei " are said to be 

 formed in this case. Whatever the correct interpretation of these " polar nuclei " 

 may be, it is important to notice that a halving of the chromosome number 

 during their formation has in no case been demonstrated. Until this has 

 been done, we cannot regard these observations as indicating that a real 

 nuclear reduction occurs in this way during gametogenesis in the organisms 

 studied. As, furthermore, the number of chromosomes present in these cases 

 previous to the so-called " reduction " appears to be an odd one, it is difficult 



H 2 



