iv THE SEX CHROMOSOMES 109 



that the failure to conjugate does not necessarily result in the sex chromo- 

 somes remaining compact at this period. 



Wilson (1912) has made some interesting observations which possibly 

 indicate a slight tendency to syndesis on the part of the sex chromosomes 

 of the male, which, however, does not culminate in actual conjugation. 

 In the case of Oncopeltus he examined a hundred nuclei in syndesis. 

 In seventy-five of these X and Y were entirely separate ; in twenty-five 

 they were side by side, just in contact. In not one case, however, out 

 of hundreds examined, were they fused or even flattened together. In 

 Lygaeus the tendency for X and Y to come together is stronger, for 

 out of a hundred synizetic nuclei forty-five showed them separate and 

 fifty-five showed them in contact (thirty-six times end to end, and 

 seventeen times side by side), often pressed together, but never fused 

 into a single body. Again, in Ascaris nigrovenosus (p. 113) Schleip 

 found that in the metaphase I. of the spermatogenesis in the hermaphro- 

 dite the XY pair were usually separate, but in one individual were 

 always, and in others rarely, united into ' a bivalent like the other 

 chromosomes. 



Another very common, but not universal, feature of the sex chromo- 

 somes is their tendency to travel to the poles of the spindle either in 

 advance of, or more often behind, the other chromosomes in the anaphase 

 of the reduction division. Here again this distinction is not found in 

 the corresponding phase in the female, except in the special case of the 

 male-producing parthenogenetic egg of Phylloxera (p. 119). Examples 

 of forms in which the X chromosomes lag in the male meiosis but not 

 in the female are Anasa, Archimerus (Wilson, 1905 ; Morrill, 1910) ; 

 Aphis and Phylloxera (Morgan, 1909 ; von Baehr, 1912). 



(5) Sex Chromosomes in Animals other than Insects 



So far, except for occasional references, we have confined ourselves 

 to the consideration of the sex chromosomes of insects, since this group 

 by itself serves to illustrate all the main variations. They have, however, 

 been found in many other forms, in some of which they can be observed 

 as clearly as in the insects already described, while in others the evidence 

 for their presence is not so satisfactory. 



Nematodes provide some very clear cases, the simplest being 

 Ancyr acanthus cistidicola, a parasite in the swim-bladder of various 

 fresh-water fish (Mulsow, 1913). The male diploid group is eleven (10 + X) 

 and the female twelve (lO + XX). During the diffuse stages of the male 

 meiotic prophase the X chromosome retains its compact character. 

 Metaphase I. effects the differential division, the secondary spermatocytes 

 having six and five chromosomes respectively. In the second meiotic 



