GENERAL DISCUSSION. 15 



suggestive of what may be necessary in the way of a combination of 

 systematic with cytological methods. 



Mendel's Law and the Individuality of the Chromosomes. 



It would hardly be possible to find better material than the germ 

 cells of the aphids to illustrate the individuality of the chromosomes, 

 and the probable working of Mendel's L,aw of Heredity. 



In every one of the twenty-four species examined some or all of the 

 chromosomes possess characteristics which distinguish them from 

 their fellows, and these peculiarities persist throughout all the gen- 

 erations. In every species where it has been possible to study and 

 compare the germ cells of the parthenogenetic and sexual generations, 

 the single series of the maturating sexual germ cells has been found to 

 be exactly duplicated in the double series of the parthenogenetic egg, 

 the segmenting winter egg, and the spermatocytes before reduction; 

 and there seems to be no room for doubt that homologous maternal and 

 paternal chromosomes are paired and then separated in maturation. 



That the spermatids must be "pure" with regard to the paternal 

 and maternal characters represented by the several pairs of chromo- 

 somes is indicated by the fact that in the spermatocyte pairing of the 

 chromosomes does not occur until the prophase of the first maturation 

 mitosis, and even then the homologous chromosomes are merely 

 paired without close union. There is therefore little opportunity for 

 mingling of the chromatin elements of any two paired chromosomes. 



In such a case as that of the Harpswell willow aphid with only 

 three chromosomes in the spermatocyte there must be extreme corre- 

 lation of characters, as only six different combinations of maternal 

 and paternal chromosomes would be possible in the mature eggs and 

 the spermatozoa, while in the maple aphid with sixteen chromosomes, 

 a large variety of different combinations of maternal and paternal 

 characters is possible. Why there should be so marked a difference 

 in number of chromosomes and in amount of chromatin in the several 

 species is at present inexplicable. I thought at first that it might 

 be possible to homologize the various series, and see where a single 

 chromosome in one species might be equivalent to two or more in 

 another, the amount of chromatin being approximately the same in 

 all. A comparative glance at the plates is enough to show that such 

 an attempt would be futile. The difference in amount of chromatin 

 in the same stage of maturation for example, the metaphase of the 

 first spermatocyte, in material treated in exactly the same way 

 though not so great as the difference in number, is still very evident. 



