STUDIES ON CHROMOSOMES 69 



connection with the spermatocyte-divisions, are thoroughly deci- 

 sive in showing the third small chromosome to be an extra ra-chro- 

 mosome not distinguishable in any respect from the other two. 



2 DISCUSSION 



It seems to me that in the individual of Metapodius that has 

 here been described nature has performed an experiment which, 

 as far as it goes, is precisely such as we should like to carry out 

 artificially in order to test the hypothesis of the genetic continuity 

 of the chromosomes and the question of their qualitative relations 

 in the maturation-process. . The experiment (if we may call it 

 such) consisted in the omission from the typical 22-chromosome 

 diploid groups of the small idiochromosome, and its replacement 

 by one of different type, a third m-chromosome. In what way 

 this was effected can only be conjectured; but it seems altogether 

 probable that the anomaly was present in the original fertilized 

 egg, as a result of one preexisting in one or both the gamete- 

 nuclei. 5 In any case we may be sure that it arose very early in the 

 ontogeny, at a period prior to the separation of the right and left 

 gonads, since both testes show precisely the same characters. 



It is certain that the initial anomaly has persisted unchanged 

 through many generations of cells, and that the alteration in the 

 diploid groups has involved corresponding modifications in the 

 maturation-process. The significant fact is that throughout this 

 process the chromosome that has been added does not take the place of 

 the one that has been omitted, but behaves according to its own kind. 

 This is a truly remarkable result when we consider that the num- 

 ber of chromosomes in the diploid groups (22) remains unaltered. 

 These groups still consist of 11 pairs of chromosomes; but one is 



5 We must assume, in this case, that the sperm-nucleus contained no small idio- 

 chromosome and that either this nucleus or the egg-nucleus contained two m-chro- 

 mosomes. The former condition may have resulted from a failure of the idiochro- 

 mosomes to separate in the second spermatocyte-division (which, as I have shown, 

 may actually occur). The presence of a second w-chromosome may be due to a 

 similar cause. 



