CHROMOSOME STUDIES 221 



as V-chromosomes the 'V simples' of Gregoire ('10, fig. 65a), 

 which occur in both anaphase and telophase of the first and in 

 the metaphase of the second maturation division (Robertson, 

 '08, figs. 37-39; Davis, '08, figs. 65, 93; the present paper fig. 

 160, nos. 7, 10, and lOx). These diads are in reality rod-shaped 

 chromosomes, equivalent to spermatogonia! rods spht longitudi- 

 nally, whose halves have a tendency to gape at the distal end. 

 The same appearance is frequently seen in spermatogonia! and 

 somatic metaphases in Syrbula, where we are certain of having 

 only rod-shaped chromosomes from which such so-called V's 

 may be derived. Frequently in illustrations of the anaphase 

 of first spermatocytes, or the metaphase of second spermato- 

 cytes, the outlines of the halves of these chromosomes are over- 

 looked, especially at the apex, so that the chromosome is made 

 to appear as a true V or U (Hartman, '13, figs. 33^7). I 

 believe that the failure of authors — Riickert and his followers, 

 who worked on copepods, and Belajeff ('98) and Ishikawa 

 ('97) on plants — properly to distinguish between the 'V simples,' 

 derived from the rod-type tetrad in anaphases of the first and in 

 metaphases of second maturation divisions, and the true V- 

 shaped type of chromosome during these stages, has led to con- 

 fusion in causing an interpretation of the second maturation 

 division as exhibiting post reduction. Furthermore, I do not 

 include among the true V's the 'V-tetrads' of insects (nos. 7, 8, 

 and 10 in fig. 157; McClung, '14, figs. 92, 93), whose V form 

 I consider to have resulted from the moving apart, along the axis 

 of the secondary split (II, fig. 156c), of the distal ends of the 

 parts of a tetrad which originally consisted of four strands lying 

 side by side (Robertson, '08, fig. 29, e and h). 



In going over the literature on these chromosomes, it has 

 seemed to me that possibly there may be two types of V's. 

 In the first the chromosome may be considered a bent rod hav- 

 ing no achromatic bridge nor marked constriction at the apex 

 of the V or U. Such chromosomes, if short, would tend to 

 be rods, straight or slightly curved; if long, they would be U's. 

 The U-shape would be especially brought out in anaphases of 

 division figures. Chromosomes of this type may be seen in 



