GENETICS 169 



There are exceptions to this. A usual sex-linked character, such 

 as color-blindness, is sometimes transmitted from father to son directly. 

 This is explained by Bridges as being due to what he terms a "non-dis- 

 junction" of the sex-chromosomes in the polar divisions of the egg dur- 

 ing the maturation division. In other words, such a non-disjunction may 

 come about by the two X-chromosomes in the egg pairing, but then 

 failing to separate, so that either both remain in the mature egg or both 

 are extruded with the polar bodies. 



In a study of parthenogenesis (virgin-birth) further evidence is 

 brought forth in regard to the function of the X-chromosome. 



For example, there may be one maturation division without a reduc- 

 tion of chromosomes. In this case the single polar-body and the egg 

 nucleus will both contain the diploid number of chromosomes. This is 

 quite common in the Crustacea and a few other forms. 



Or, there may be two polar divisions, after which one of the polar 

 bodies reunites with the egg nucleus. Here, again, the full number 

 {diploid number) of chromosomes are found. 



Or, in some forms (Hymenoptera and the male-producing eggs of 

 Rotifers) two polar divisions really take place, which reduce the chro- 

 mosomes to the haploid number. If these eggs are unfertilized, they 

 give rise to males. Such eggs already have only half the full number 

 of chromosomes. Consequently in their germ cells, in turn, there is no 

 further reduction. The first spermatocyte division in these is really 

 suppressed. If the eggs are fertilized they produce females. 



In those Hymenoptera where there are two divisions, but the chro- 

 mosomes divide at the equator and not longitudinally, the diploid num- 

 ber is retained and females are usually produced. 



Various other evidences of great value and interest will be found 

 in the books on Cytology mentioned at 'the end of this chapter. 



The diagram of the chromosome-cycle of Phyllaphis coweni (Fig. 

 83) will throw light on this subject. 



The top group shows a fertilized egg with four ordinary chromo- 

 somes and two X-chromosomes. Three lines of descent pass downward 

 from the egg. On the left, this line of descent leads to a female which 

 will produce a sexual egg. The central line of descent leads to a female 

 which will reproduce parthenogentically, and on the right the line of 

 descent leads to a male. 



The second and third groupings from the top represent the meta- 

 phase groups as well as the diagrammatic anaphases of three eggs, of 

 which the left and middle will produce females, the right a male. In 

 the female-producing eggs, the X-chromosomes divide at the equator 

 and not longitudinally ; while in the male-producing eggs they pair and 

 separate so that the male has only one X-chromosome. 



The fifth grouping from the top at the left, is the metaphase group 

 of the first polar division of the sexual egg. All the chromosomes are 



