io8 CYTOLOGY 



CHAP. 



follows that in the female, which has two equivalent X's, these should 

 participate in syndesis, and that therefore they should not have the 

 compact form noticeable in the male at this stage. While the female 

 meiotic phase has not been so carefully studied as the male, the facts, so 

 far as they are known, are in general accordance with this hypothesis. In 

 the female syndetic and meiotic prophase nuclei in, e.g., Anasa, Harmostes, 

 Alydus, Euschistns, Coenus and Podisus among Hemiptera (Wilson, 

 1906 b) and Ancyr acanthus in Nematodes (Mulsow, 1913), the sex chromo- 

 somes are as extended and diffuse as the others, though they are compact 

 in the corresponding stage in the male meiosis of these species. 



Another fact bearing out the same view is, that the X and Y chromo- 

 somes are not usually compact in the oogonial, spermatogonia! and 

 somatic prophases, even in those species where they are so in the male 

 meiotic prophase, e.g., Anasa, Harmostes, Alydus, Euschistns, Coenus 

 and Podisus (Wilson, 1906 b), Archimerus, Anasa, Protenor, Chelinidea 

 (Morrill, 1910), Euschistus (Montgomery, 191 1). 



A more striking example is afforded by the hermaphrodite generation 

 of Ascaris nigrovenosus (p. 113). This is of the female build of body, 

 but produces both eggs and spermatozoa in its reproductive organ. 

 The diploid group is 2n' +XX, where «' =5. The primitive germ cells, at 

 first all alike, differentiate later into oogonia and spermatogonia. In 

 the oogenesis the XX pair acts exactly like the other bivalents. In the 

 male meiotic prophase, however, one of the X's may be said to take on 

 the characteristics of a Y chromosome. This XY pair condenses out 

 sooner than the other chromosomes, though in the female meiosis, where 

 both sex chromosomes retain their X character, they do not do so. 



In some species {Aphis, Fig. 53) the X chromosome is fihform like the 

 others during syndesis, although, there being no Y chromosome, it cannot 

 be participating in this process. Even here, however, the X chromosome 

 is clearly distinguishable from the others in the diplotene and later 

 stages, being single or inconspicuously spUt for the second meiotic 

 division, unlike the conspicuously double bivalents formed by syndesis 

 of the other chromosomes (Fig. 53, E, F, G). 



Taking everything into consideration, therefore, it is hard to escape 

 from the conclusion that in these species (by far the majority) where 

 the sex chromosomes remain compact during syndesis and the rest of 

 the meiotic prophase in the male, this is an expression of the fact that 

 they are not themselves engaged in the act of conjugation. It must be 

 admitted, however, that compactness at a time when the other chromo- 

 somes are Unear or diffuse is not always due solely to the fact that the 

 one set is engaged in syndesis and the other not, for in some species the 

 same difference between the consistency of the sex and ordinary chromo- 

 somes is found outside the meiotic phase. The case of Aphis also shows 



