114 CHARLES V. MOKRILL. 



in the male and female pronuclei was not determined with cer- 

 tainty but is probably 24 in each. On the whole, the chromo- 

 somes were small and the size-differences not well marked. 



Hemiptera-homoptera. The older papers of Weismann, Wit- 

 laczil, Blochmann and Will on the early embryology of aphids 

 contain no detailed account of the chromosomes. Stschelkanov- 

 zew ('04) in a brief paper on the maturation and early cleavage 

 of the summer (parthenogenetic) egg of Aphis roses, gave 14 as 

 the number of chromosomes in the maturation spindle (only one 

 polar body is formed). In one first cleavage spindle there were 

 only 1 1 chromosomes but he considered that three of these might 

 be double elements, thus giving the diploid number, 14, in both 

 maturation and cleavage. Miss Stevens ('050) and Hewitt 

 ('06), 15 however, found the diploid number in the parthenogenetic 

 egg of Aphis roses, to be 10 and this count has been confirmed by 

 von Baehr ('09) in the same species. Miss Stevens also found 

 that the winter (fertilized) egg gave off two polar bodies in which 

 the haploid number, 5, was present. Three of these authors 

 observed marked size differences in the chromosomes of the ma- 

 turation and early cleavage stages, both Miss Stevens and von 

 Baehr finding four smallest chromosomes constantly. Miss 

 Stevens ('o6fr) in a very extensive paper described the maturation 

 and cleavage in a large number of aphids, with especial reference 

 to the number and behavior of the chromosomes. Without 

 giving a detailed review of her results it may be said in general 

 that the number and size differences of the chromosomes was 

 found to be constant for the species and that this constancy 

 applies to the diploid groups whether in the maturation spindle 

 of a parthenogenetic egg or in its cleavage spindles. In many 

 cases also the haploid group was found to exhibit the same rela- 

 tive size differences as the diploid group of the same species. 

 In the fertilized egg of the "Goumi aphid," the number of chromo- 

 somes in the male and female pronuclei just before copulation 

 was shown to be 5 in each, i. e., the haploid number. Mi^ 

 Stevens concluded from her observations on aphids up to this 

 time that there were no "heterochromosomes" in this group, but 



"Hewitt's results are known to me only through the brief mention made l>\ vmi 

 Baehr ('09, p. 285). 



