COMPARISON OF RESULTS. 5 1 



COMPARISON OF RESULTS IN DIFFERENT SPECIES OF 

 COLEOPTERA. 



In number of chromosomes there is great variation, the smallest 

 number (i6) having been found in Odontota dor sails, and the largest 

 (40) in Silpha americana. The difference in size is also very marked, 

 as may be seen by comparing the spermatogonial plates in figures 3 

 and 58 with those shown in figures 94 and 141. 



No other species of the Tenebrionidae has yet been secured, and all 

 of the other beetles examined differ in a marked degree from Tenebrio 

 molitor in the growth stages of the spermatocytes. While in Tenebrio 

 the chromatin stains very dark throughout the growth stage, and the 

 unequal pair can not be distinguished until the prophase of division 

 ('05, plate VI, figs. 171-180), in most of the others there are very dis- 

 tinct synizesis and synapsis stages, following the last spermatogonial . 

 mitosis, then a spireme stage in which the condensed unequal pair of 

 heterochromosomes or the odd chromosome is conspicuous in contrast 

 with the pale spireme, whether the preparation is stained with iron- 

 haematoxylin , gentian, or thionin. In Tenebrio molitor, the unequal 

 pair behaved in every respect like the other bivalent chromosomes. 

 In the other forms, though it behaves during the two maturation divi- 

 sions like the symmetrical bivalents, it remains condensed during the 

 growth period like the "accessory" of the Orthoptera, the odd 

 chromosome, " »?-chromosomes," and " idiochromosomes " of the 

 Hemiptera. In several cases the heterochromosomes of the Coleoptera 

 are associated with a plasmosome (figs. 22, 23, 63, 132, 158, 217), as 

 is often true in other orders. This peculiar pair of unequal hetero- 

 chromosomes varies considerably in size during the growth stage in 

 some of the species studied, but changes very little in form, differ- 

 ing in this respect from the ' ' accessory ' ' in some of the Orthoptera 

 (McClung, '02) and from the large idiochromosome in some of the 

 Hemiptera (Wilson, '05). 



The odd chromosome, so far as it has been studied, behaves pre- 

 cisely like the larger member of the unequal pair without its smaller 

 mate (figs. 219, 220, 226, 233). In the growth stage it remains con- 

 densed and either spherical or sometimes flattened against the nuclear 

 membrane (figs. 217, 225, 231). In the first maturation mitosis it is 

 attached to one pole of the spindle, does not divide, but goes to one of 

 the two second spermatocytes (figs. 233, 235). In the second sper- 

 matocyte it divides with the other chromosomes, giving two equal 

 classes of spermatids differing by the presence or absence of this odd 

 chromosome. 



