THE GERM-CELLS OF CICADA (XIBICEN) SEPTEMDECIM. 413 



tetrad is produced. Payne ('14) has described a different method 

 of ring-formation in Forficula. The two univalent chromosomes 

 are first joined end-to-end; while retaining this point of union, 

 the free ends come together by a bending process with the result- 

 ing formation of a ring each half of which represents a univalent 

 chromosome. A similar method of ring-formation has been 

 described by Sutton ('02) in Brachystola and by Davis ('08) in 

 several Orthopterans. In Forficula besides the "bending pro- 

 cess" of ring-formation, Payne describes ring-formation of the 

 type here described in Cicada. 



(r) Maturation Divisions. In the metaphase plates of the 

 first maturation division, the chromosomes are always grouped 

 in a characteristic manner (Figs. 23, 24, 53 to 57). There are 

 10 chromosomes in the metaphase plate, 8 of which are arranged 

 in a circle surrounding the macrochromosome tetrad (A A , Fig. 23) 

 which always lies in the center of the group. The sex chromosome 

 (Fig. 23, X) always lies outside this circle of chromosomes and 

 often does not lie in the same plane. Boring ('07) has figured the 

 chromosomes of a number of species of Homopterans in which 

 the sex-chromosome lies outside the group of autosome tetrads. 

 The constant position of the AA tetrad in the center of the 

 spermatocyte complex can be traced back to the diploid chromo- 

 some groups in which the macrochromosome pair shows a marked 

 tendency to lie in the middle of the metaphase plate with the 

 other chromosomes grouped about them (text-fig. 2). In a 

 previous paper (Shaffer, '20) it was pointed out that the char- 

 acteristic grouping of the chromosomes in the metaphase had its 

 explanation in the persistence of the interchromosomal linin 

 fibres (Fig. 24) which undoubtedly persist as a part of the chro- 

 mosomal architecture and maintain definite spatial relations 

 between the chromosomes. The evidence in Cicada seems to 

 support this view. 



In the side-view of the metaphase, the AA and BB tetrads 

 are arranged on the spindle in the direction of the spindle axis. 

 Hence in polar views only a half of each tetrad can be seen (Fig. 

 23). In both the A A and BB tetrads the spindle fiber attach- 

 ments are median (Figs. 18, 19) or atelomitic (Carothers, '14), 

 and since the tetrads lie in the direction of the spindle axis, they 



