332 N. M. Stevens. 



DESCRIPTION OF PLATES. 



.With the exception of Figs. 20 and 21, the figures are all camera drawings, made with Zeiss obj. 

 2 mm., oc. 8, except Figs, i, 14, 15 and 19, which were drawn with Zeiss obj. 2 mm.,oc. 4, and Fig. 22, 

 for which Zeiss ohj. D,oc. 4 was used. Plates II and III have been reduced one-tenth. 



Plate I. 



Fig. I. Parthenogenetic ovary from an embryo of Aphis rosae. a = a developing ovum, fe = stalk 

 of the next ovum in the egg-string. 



Fig. 2. Oocyte from above ovary. 



Fig. 3. Oocytes at the beginning of their growth period, showing chromosomes but no nucleolus. 



Figs. 4-6. Later growth stages of the oocyte. 



Fig. 7. Equatorial plate of the polar spindle, showing 10 chromosomes of 5 different sizes and 

 shapes a-e. 



Fig. 8. Polar spindle in anaphase. Polar body already separated from the egg. 



Fig. 9. A later stage, showing the polar body (/>), and the egg-nucleus (n). 



Fig. 10. An egg containing the first segmentation spindle, and the polar body (p) somewhat sunken 

 in the cytoplasm of the egg. 



Fig. II. Segmentation stage with resting nuclei and polar body (p) lying in a vacuole in the cyto- 

 plasm of the unsegmented egg. 



Fig. 12. Segmentation stage, showing polar body (/)) and mitotic figures. The chromosomes of 

 the equatorial plate are lettered a-e to correspond to those cf tht; equatorial plate of the polar spindle 

 in Fig. 7. 



Plate II. 



Fig. 13. Later segmentation stage, showing polar body (/>) probably degenerating. 



Fig. 14. Young parthenogenetic embryo just beginning to take in yolk (a) through a valvular 

 opening in the oviduct (b). 



Fig. 15. A slightly later stage of the same process. 



Fig. 16. Resting oocyte from an ovary of Aphis cenotherae, all the embryos male. 



Fig. 17. Section of a segmenting egg from an individual containing male embryos, showing one 

 polar body (p). 



Fig. 18. Part of a section similar to the above, showing chromosomes not so completely fused. 



Fig. 19. Ovary from a female embryo of the sexual generation, showing degenerating oocytes (a). 



Fig. 20. Freehand sketch of a mixed group of ovaries, consisting of five winter ovaries (a) with 

 winter eggs (b), and one parthenogenetic ovary (c) with developing eggs and embryos (e). 



Fig. 21. Freehand sketch of egg-string from the other side of the same individual, showing two 

 parthenogenetic embryos (e), a winter egg (b) and a winter ovary (a). 



Plate III. 



Fig. 22. Oblique section of a winter egg, showing entrance of secondary yolk from without, a = 

 yolk cell (nucleus in next section), b and c = fragments of nuclei. 



Fig. 23. Equatorial plate of first polar spindle in winter egg of Aphis rosae. The five chromosomes 

 are lettered a-e corresponding to the five pairs in Figs. 7 and 12. 



Fig. 24. First polar spindle, metaphase. The stippled chromosome is a part of one which appears 

 in a. 



