8 STUDIES ON THE GERM CELLS OF APHIDS. 



The Milkweed Aphids. 



On the milkweeds at Woods Hole, only one kind of aphid was 

 found, a very dark colored one nearly black which lived in herds 

 attended by ants. Sexual forms were collected on September 27. The 

 metaphase of the maturation mitosis of the parthenogenetic egg is 

 shown in figure 44, and that of a spindle of the 4-cell stage in figure 

 45. The prophase of a spermatogonial mitosis appears in figure 46, 

 exhibiting the same double series of chromosomes as in the parthen- 

 ogenetic egg. The metaphase of the first spermatocyte is shown in 

 figure 47, and daughter plates of the anaphase in figure 48. The 

 lagging chromosome in the anaphase (fig. 49) is here the smallest of 

 the four. Figures 50 and 51 show different views of the anaphase of 

 the second spermatocyte. The number of chromosomes is the same 

 as in the second Oenothera aphid, but the difference in form and size 

 is evident. (Compare figs. 38 and 44, 39 and 47.) 



On the milkweeds at Bryn Mawr two species were found in abun- 

 dance, one pale (white with some brown spots), and the other a bril- 

 liant orange. The latter remained parthenogenetic until the plants 

 were destroyed by frosts, no sexual forms appearing. A few drawings 

 were made from the parthenogenetic egg. Figures 52 and 53 are the 

 metaphase of the maturation spindle, with two chromosomes conspic- 

 uously larger than the other six. Figure 54 is a similar stage, but 

 the two largest chromosomes are evidently united. Two equatorial 

 plates of this kind were found in embryos of one mother, while the 

 usual number eight appeared in segmentation stages of eggs in the 

 same embryos. In this species the chromosomes become distinct and 

 take stains well from the moment when the growth stage of the oocyte 

 begins. A young oocyte, just leaving the ovary, is shown in figure 



55. A large plasmosome (p) and six of the chromosomes appear in this 

 section. A prophase of mitosis in a i6-cell stage is shown in figure 



56, and a section of a 4-cell stage, showing the polar body (pb] and 

 the metaphase of three spindles, in figure 57. 



In the spermatocytes of the pale milkweed aphid there are seven 

 chromosomes, 'always grouped in a characteristic way (figs. 58 and 

 59). The smallest chromosome is closely associated with the next in 

 size and the two occupy the center of the plate. The arrangement 

 suggested the conditions in some of the Hemiptera heteroptera, but 

 careful examination of the anaphase of both maturation spindles 

 showed that these two chromosomes divide like the others, both, how- 

 ever, lagging behind the others (figs. 60 and 61). Figure 62 is a 



