324 N. M. Stevens. 



each recurrence ot sexual reproduction, giving germ cells pure 

 with respect to each ot the 5 different characters or sets of corre- 

 lated characters represented by the five chromosomes. During 

 the whole series of parthenogenetic generations the same paternal 

 and maternal series of chromosomes is maintained by longitudinal 

 division, there is no amphimixis and no apparent chance for varia- 

 tion unless it be a change in dominance of certain characters, due 

 to external conditions; for example, the winged-character and the 

 sex-character. 



The constant recurrence of this sm^le or double series of chro- 

 mosomes of the same relative form and size, is one point more in 

 support of the hypothesis of the individuality of the chromosomes, 

 strongly advocated by Rabl ('85) and Boveri ('87, '88, '91, '02). 

 Recent papers bv Sutton ('02) on Brachystola and Baumgartner 

 ('04) on Gryllus show similar form and size relations ot chromo- 

 somes; but in the Aphid one has the advantage of working with a 

 smaller number, where each individual chromosome can be dis- 

 tinguished from all others of the series appearing in a winter egg 

 or a spermatocyte. 



2. Maturation of Parthenogenetic Eggs. 



In comparing the results of various authors on this subject, one 

 meets with great variations in different parthenogenetic forms. 

 In the drone bee (Blochmann, '88-'89) two polar bodies are found, 

 and Petrunkewitsch ('01) states that reduction occurs in the second 

 maturation division, the normal number of chromosomes probably 

 being restored by a subsequent longitudinal splitting ot the chro- 

 mosomes without mitosis. In Liparis dispar, Bombyx mori and 

 Ocneria dispar (Platner, '88-'89, Henking, '92), two polar bodies 

 are given off by the occasional parthenogenetic eggs and both sexes 

 are produced. Weismann ('91), in attempting to bring these cases 

 into line with his views of maturation and fertilization, says, "The 

 nucleoplasm of certain eggs possesses a greater power of growth 

 than that of a majority of the eggs of the same species, while in the 

 case of the bee every ovum possesses a power of growth sufficient 

 to double its nuclear substance." Petrunkewitsch's explanation 

 of the presence of the normal number of chromosomes in somatic 

 cells of the male bee sounds more like an echo of Weismann's 

 ar2;ument than like the result of actual observation. 



