PREDETERMINATION OF SEX 299 



seen only the early prophases of the first division or possibly the 

 stage between the first and second spermatocytes. 



The side view of the first spermatocyte division is shown in 

 plate 2, figure 6. An equatorial plate is shown in plate 2, fig- 

 ure 5, which gives the relative sizes of the three chromosomes. 



Stages in the later first division, some of them duplicates of 

 each other, are shown in figures 7 to 11. The two autosomes di- 

 vide; the X chromosome is drawn out, and is usually dumb-bell- 

 shaped. It generally shows a well marked longitudinal split. 

 The split is so distinct that the halves appear often like two 

 parallel lagging chromosomes. Only in the latest stages of the 

 division is it apparent that the whole X chromosome passes into 

 the larger of the two cells. As in other aphids, the X chromo- 

 somes are, during this division, often constricted near the middle, 

 which in some species is sometimes carried so far that the two 

 enlarged ends are connected by a mere strand. It is this ap- 

 pearance that has led Miss Stevens in her earlier work to infer 

 that the X chromosomes were really divided at this time. In 

 the aphid of the bearberry the constriction appears at first at 

 the middle of the chromosome. It seems later to pass more and 

 more towards one end, until ultimately, as shown in figures 11 

 and 13, a small piece only is left at one end, which in most cases 

 is later drawn into the thread; although once or twice I have 

 found cases, as in figure 12, in which this terminal piece appears 

 to have broken away from the strand connecting it with the 

 rest. 



The difference in size of the two cells varies greatly in the 

 earlier phases, as the figures show. But ultimately nearly all 

 of the protoplasm passes into one cell, the one that contains the 

 X chromosome. The small cell is left with two chromosomes 

 and a small amount of cytoplasm. It never divides again, and 

 later degenerates. Stevens was inclined to think that the small 

 cell may sometimes show a division figure, which subsequently 

 fades away, but I have never seen a case of this kind. The two 

 autosomes in the functional cell begin to lose their condensed 

 condition and spread out into loose masses, as shown in figures 

 16 and 17. The X chromosome is later in passing into this con- 



