IV THE SEX CHROMOSOMES 117 



the cell (secondary spermatocyte) containing the X chromosome being 

 larger than the other one and, moreover, receiving the whole of the 

 chondriosomes. The larger cell proceeds to the second meiotic division 

 in the usual way, the X chromosome dividing this time so that both the 

 resulting spermatids have an X chromosome. On the other hand, the 

 smaller spermatocyte II. without the X chromosome proceeds as far 

 as prophase II. (Fig. 53, M), but degenerates without completing the 

 second division. In other words, only the one class of spermatozoon, 

 namely, the X-bearing or female-producing ones, are formed. Hence it 

 is clear why all fertilized eggs develop into females. 



(2) The second problem is not completely cleared up by von Baehr's 

 account for Aphis. The same individual gives rise parthenogenetically 

 to both males and females, so that presumably the mature egg is some- 

 times left with five chromosomes (male-producers) and sometimes with 

 six (female-producers) ; indeed, segmenting eggs and embryos are found 

 to have sometimes six and sometimes five chromosomes in their nuclei. 

 The mechanism by which one chromosome is eliminated in the formation 

 of the male-producing egg could not, however, be determined in this 

 animal. The ehmination of the second X chromosome during the 

 maturation of the male-producing eggs was, however, observed in the 

 following case. 



Phylloxera (Morgan, 1909, 1915 a ; Fig. 54). The life history of this 

 genus is the same in principle as that of Aphis. All fertilized eggs give 

 rise to females (stem mothers) which hatch in the spring. These produce, 

 parthenogenetically, other females which in turn produce parthenogenetic- 

 ally males and sexual females. The eggs which will develop into males 

 are smaller than those which will develop into females. In Phylloxera 

 caryaecaidis all the daughters produced (parthenogenetically) from one 

 stem mother produce the same kind of egg — i.e. all are either male- 

 producing or female-producing. In the case of P. fallax both kinds of 

 daughters appear to be produced from the same stem mother, and 

 perhaps both kinds of sexual eggs from the same daughter. 



Taking the case of P. fallax, which presents fewest complications 

 (conditions in P. caryaecaidis being the same in principle), the diploid 

 number is twelve in the female and ten in the male, the X chromosome 

 consisting of two components. The male may therefore be represented 

 by the formula 8 + X2 and the female by 8 -l- X.,X._,. The two components 

 of the X chromosome act as a single compound chromosome as in 

 Syromastes, etc. 



Spermatogenesis proceeds in the same way as in A . saliceti ; in 

 anaphase I. the two X components pass intact into one secondary 

 spermatocyte, which is larger than its sister cell, and alone proceeds to 

 the second division ; thus all spermatozoa are female-producers. 



