6 STUDIES ON THE GERM CELLS OF APHIDS. 



The Willow Aphids. 



In some material collected by Prof. Morgan from willow trees at 

 Saranac, New York, early in September, were found a few young males 

 and mothers of the sexual generation. Figures 20a and 20b are two 

 sections of a first spermatocyte before the pairing of the chromosomes, 

 each section containing one complete series of five. Figure 21 is a 

 similar stage showing the two largest chromosomes and five of the 

 smaller ones. Figure 22 shows two pairs of homologous chromosomes 

 from a slightly later stage than that of figures 20 and 2 1 . Figure 23 

 is the metaphase of the first spermatocyte, and figure 24 shows daugh- 

 ter plates of the anaphase. In figure 25 is seen a pair of second sper- 

 matocytes immediately after the formation of the nuclear membrane. 

 The second division was not found, but a drawing of the young 

 spermatid is given in figure 26. The number of chromosomes here is 

 the same as in the brown rose aphid, but the proportionate size is 

 somewhat different. There is more difference in size between the 

 largest and the second in size, and the two smallest are proportion- 

 ately larger than the corresponding ones in the rose aphid. The 

 mothers of the sexual generations produce offspring of both sexes. 



Much to my surprise, material collected on June 29 at Harpswell, 

 Maine, contained sexual forms. These aphids were taken from the 

 hairy, reddish young shoots of one of the dwarf willows, and they 

 could not have been many generations away from the winter egg, for 

 it was still early spring on the coast of Maine. There were a few 

 young males and females, and the apterous parthenogenetic mothers 

 contained embryos in the following combinations : 



1. All parthenogetic. 



2. All sexual female. 



3. Parthenogenetic and sexual female. 



4. Parthenogenetic male. 



5. Parthenogenetic male and female. 



Kyber (18 15) mentions finding sexual forms on the willow the last 

 of June, but attributes their presence to the change in sap conditions 

 of the host at the close of the growing season, while in this case they 

 were found on young growing shoots. 



The polar spindle of the parthenogenetic egg in metaphase is shown 

 in figure 27, and prophases of segmentation spindles in figures 28 and 

 29. Six chromosomes, which evidently form three pairs, are present. 

 Figure 30 shows resting cells of a segmenting parthenogenetic egg. 

 The conspicuous nucleolus with its several vacuoles divides into two 

 as a prophase of nuclear and cell divisions. Figures 31 and 32 are 

 first spermatocytes in metaphase and figures 33 and 34 in anaphase; 



