74 



N. M. STEVENS. 



second in size ('09, PI. II., Figs. 54, 56, 57^) and here it is evi- 

 dently the two second in size, which have fused to form the 

 large vacuolated chromosome x. No prophases or anaphases 

 were found. 



In the young male embryos many cells were in mitosis and in 

 a few cases it was possible to count and draw the chromosomes. 

 Fig. 3 is a nucleus in prophase, flattened so that the nine chromo- 

 somes are nearly in the same plane. It will be seen that the two 

 longest form a pair, while the next in size is unpaired. Fig. 4 is 

 a metaphase from another embryo. The unpaired chromosome 

 is again the second in size. 



This evidence, so far as it goes, indicates that one whole 

 chromosome goes into the polar body of the male &%,%, leaving 



Fig. 5. Male egg showingthe single polar body (/) and two nuclei. Zeiss 1.5-2. 



the somatic number for the male individual reduced by one. 

 Ordinarily one would not lay much stress on the conditions ob- 

 served in a single egg, but considering the difficulties involved in 

 securing favorable stages of maturation in male eggs of aphids, 

 and the fact that the observations are in accord with Morgan's ^ 



1 " An Unpaired Heterochromosome in the Aphids," Journ. Exp. ZooL, Vol. 

 VI., No. I, Jan., 1909. 



2 " A Biological and Cytological Study of Sex Determination in Phylloxerans and 

 K^hids," Journ. Exf. ZooL, Vol. VII., No. 2, Sept., 1909. 



