STUDIES ON CHROMOSOMES 
69 
connection with the spermatocyte-divisions, are thoroughly deci- 
sive in showing the third small chromosome to be an extra 7n-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^enetic 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.^ In any case we may be sure that it arose very earlj^ 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 anomal}' 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 imm- 
ber of chromosomes in the diploid groups (22) remains unaltered. 
These groups still consist of 11 pairs of chromosomes ; but one is 
^ We must assume, in this case, that the sperm-nucleus contained no smallidio- 
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 j. The presence of a second m-chromosome may be due to a 
similar cause. 
