Further Studies on the Chromosoines 



103 



be called the synizesis stage, shows the odd chromosome {x) still 

 condensed, and the others forming a rather fine and closely 

 wound spireme (Fig. 7). The fine spireme of Fig. 7 gradually 

 thickens and spreads out (Fig. 8), and later loses much of its 

 staining quality (Figs. 9 and 10). In these later growth stages 

 the spireme w^nds in such a way as to appear in tangential sec- 

 tion (Fig. 10) to radiate from the heterochromosome {x). There 

 seems to be no question in this case but that synapsis must occur 

 in the stage shown in Figs. 5 and 6, since the spireme, once 

 formed (Fig. 7), remains unbroken until the prophase of the first 

 maturation division. 



When the chromosomes come into the first spermatocyte spindle, 

 nine of them are often typical tetrads, and one a dyad (Fig. 11). 

 A comparison of x which is univalent with the bivalents con- 

 vinces one that this is a reducing division for the bivalents and 

 quantitative for the odd chromosome. In early metaphase 

 the chromosomes viewed from one pole of the spindle are circular 

 or oval in outline (Fig. 12), but in metakinesis and anaphase 

 (Fig. 13) nine are dumb-bell shaped and one, the heterochromo- 

 some, is circular. Figs. 14 and 15 show the typical metakinesis 

 and early anaphase, while Fig. 16 is a late anaphase. 



The second spermatocytes all contain ten chromosomes, of 

 w^hich one, the daughter heterochromosome (x) usually stands out 

 to one side of the equatorial plate (Fig. 17), and nearer one pole 

 of the spindle in metaphase and anaphase (Figs. 18 and 19). 



In most of the Coleoptera w^here an odd chromosome has been 

 found, it passes undivided to one pole of the first spindle, and 

 divides in the second division, as in the Orthoptera and many Hem- 

 iptera homoptera; but in Photinus we have a case like that of Anasa 

 and several other Hemiptera heteroptera where the unpaired 

 chromosome undergoes its quantitative division in the first 

 spermatocyte while its bivalent companions are being separated 

 into their univalent elements. 



In Photinus consanguineus the number of chromosomes is 

 the same as in Photinus pennsylvanicus, twenty in the female 

 and nineteen in the male. The chromosomes of the daughter 

 plates are easily counted after the cell has divided (Fig. 20), 



