Febeuaet 5, 1909] 



SCIENCE 



235 



2. The females that result from the fertil- 

 ized egg produce subsequently both males 

 and females parthenogenetically. Clearly the 

 egg as well as the sperm contains factors that 

 determine sex. I have found, in fact, that the 

 same method used by the sperm is also made 

 use of by the egg. 



3. I wish to raise certain questions con- 

 nected with the mechanism by means of which 

 males and females are produced. It has gen- 

 erally been assumed that chance alone de- 

 termines into which spenn the sex chromo- 

 some passes. There are indications in the 

 egg that this mechanism is not a chance re- 

 sult, but that behind it lie a series of pre- 

 liminary events that are equally to be reckoned 

 with as sex-determining factors. 



The life cycle of a typical species of phyl- 

 loxeran consists of the stem-mother (arising 

 from the fertilized winter egg), of a second 

 winged generation (produced by the stem- 

 mother), and of sexual males and females — 

 mere pigmies in size — ^that come from the 

 winged individuals. The winged individuals 

 are of two kinds, those bearing large eggs that 

 produce the sexual females, and those bearing 

 small eggs that produce the males. Certain 

 species deviate from this rule. I shall refer 

 to one such species later. 



Phylloxera fallax has the following chromo- 

 somal history. The polar spindle of the egg 

 laid by the stem-mother has twelve chromo- 

 somes. One polar body is extruded. Twelve 

 chromosomes remain in all of the eggs and are 

 found in the somatic cells of the winged gen- 

 eration. 



The polar spindle of the female egg laid by 

 a winged individual contains twelve chromo- 

 somes. One polar body is extruded, and 

 twelve chromosomes remain in the egg to give 

 this number to the somatic cells of the sexual 

 female. 



The polar spindle of the smaller male egg 

 also contains twelve chromosomes. One polar 

 body is given off. Ten chromosomes remain 

 in the egg, and this number characterizes all 

 of the body cells of the male, as well as the 

 spermatogonia! cells. Evidently two chromo- 



somes have disappeared, when the polar body 

 is produced. 



In the first spermatocytes six chromosomes 

 appear. At the first division four of these 

 dividte equally, but two do not divide; they are 

 the accessories or sex chromosomes and pass 

 to one cell. This cell becomes the functional 

 sperm; the other, containing four chromo- 

 somes, degenerates. At the second spermato- 

 cyte division all six chromosomes in the 

 functional spermatocyte divide equally, so 

 that each of the two spermatozoa gets six 

 chromosomes.^ 



The sexual egg contains a polar spindle of 

 six chromosomes. Presumably two polar 

 bodies are formed and six chromosomes re- 

 main in the pronucleus. 



With these facts we can reconstruct the life 

 cycle of the chromosomes. 



Egg 6 Sperm 6 



\ / 



Stem Mother 12 



Polar body 12 



Female Producer 12 Male Producer 12 



Polar body 12 Polar body 12 



Sexual Female 12 



Male 10 



Sexual Egg 6 Functional Sperm 6 



Before commenting on these results I wish 

 to call attention to another species. Phyllox- 

 era carywcaulis. 



In this species there are six chromosomes 

 in the polar spindle of the egg laid by the 

 stem-mother. One polar body is extruded. 

 The somatic cells of the embryos contain six 

 chromosomes. 



The polar spindle of the female egg con- 

 tains six chromosomes. Six are given off in 

 the single polar body, and six remain in the 



The polar spindle of the male egg also 

 contains six chromosomes, hut they are of dif- 

 ferent sizes from those in the female egg. 



^ In the aphids also two kinds of spermatozoa 

 are formed. The accessory — single in this case — 

 passes into only one of the spermatocytes. This 

 fact I determined in the winter of 1907-8 and 

 von Baehr and Stevens have also reached the same 

 conclusion (1908). 



