The Germ Cells of Aphis. 319 



the greenhouse January 23, 1904, on a small rose bush which had 

 lost its leaves and was nearly dead as a result of serving as a food 

 plant for several generations of Aphids. They were the third 

 generation from a winged female. Two or three others escaped 

 dissection. Unfortunately only freehand sketches were made, 

 and no other such cases were observed. One of these sketches 

 is reproduced in Fig. 20 and a part of another in Fig. 21. Similar 

 observations were recorded by Bonnet ('45) and by Leydig ('50), 

 and Kyber ('15) observed sexual forms on the willow in June and 

 on ripening grain in midsummer, but did not connect both par- 

 thenogenetic young and winter eggs with the same individual. 



The oocytes in the winter ovaries increase immensely in size 

 before any eggs are given off, and the large nearly spherical ova- 

 ries are easily distinguished in dissections from the minute oval 

 parthenogenetic ovaries (Fig. 20, a and r). The first egg is given 

 off after the birth of the sexual female. 



It is not my purpose to describe the growth period of the winter 

 egg in detail. The nucleus, at first central, gradually moves to the 

 periphery at one side, usually nearer the anterior end of the egg 

 as it lies in the oviduct. When the egg has reached about one- 

 half its ultimate size, yolk material from the vitellaria is 

 taken in at the posterior end of the egg. There seems to be 

 no such definite opening as in the case of the parthenogen- 

 etic embryo and of the eo-g; of Aleurodes. Fig-. 22 is an 

 oblique section through an egg which plainly shows the rela- 

 tion between the yolk within and that without the egg. Entrance 

 seems to be effected between any of the cells of the follicle epithe- 

 lium, which is merely a part of the oviduct. Here again one 

 wonders which is the active agent, the egg or the yolk cells, but 

 it is impossible to tell. In the case of the embryo one is inclined 

 to believe that it may actively suck in the yolk as the planarian 

 embryo does, but there is nothing to indicate that the egg could 

 have any such power. In the case of Hydatina senta, Lenssen 

 ('98) describes the yolk cells as penetrating the egg by their own 

 activity. The entrance of yolk seems to be associated with a 

 definite size of the ovum and would therefore occur at a definite 

 point in the oviduct. The relation between the oviduct and the 

 yolk gland may therefore be a more definite one than the sections 

 show. Entrance of yolk must be effected very quickly as cases 

 where the process can be demonstrated are very rare; and one 



