578 THE AMERICAN NATURALIST. [Vol. XXXIX. 



appears during the formation of the tetrad simply separates the 

 two somatic chromosomes of the pair, while the longitudinal 

 fission is the' usual division of chromosomes, appearing prema- 

 turely at this time. The conclusion is natural that the two 

 series of the 1 1 pairs consist of maternal and paternal chromo- 

 somes which are distributed as organic entities by the reducing 

 divisions. But there are no reasons for supposing that all of 

 the paternal chromosomes will pass into one set of germ cells 

 and all of the maternal into another but rather that the ratios 

 of distribution will be by the law of chance according to which 

 the great majority of germ cells will have a marked preponder- 

 ance of chromosomes from one parent or the other, and will 

 therefore be relatively pure. An exceedingly small proportion 

 of germ cells may, by the law of chance, contain chromosomes 

 entirely of maternal or paternal extraction, and an equally small 

 proportion may contain 6 chromosomes of one parent and 5 of 

 the other. The accessory chromosome divides but once during 

 the mitoses of spermatogenesis so that two of the spermatozoids 

 have 1 1 chromosomes and two 1 2 . No accessory chromosome 

 appears in the mitoses of oogenesis indicating that the female 

 insect lacks this structure which confirms the belief of McClung 

 (: 02) and others that the accessory chromosome is a determin- 

 ant of the male sex. 



Montgomery in a series of studies upon insects and Amphi- 

 bians, which are summarized in a recent paper (: 04), reached 

 conclusions in striking support of the theories of the individu- 

 ality of the chromosomes, the association of pairs of chromo- 

 somes during synapsis to form bivalent structures and the prob- 

 ability of the elements of each pair (bivalent chromosomes) being 

 of maternal and paternal origin respectively. His results on the 

 last point are of especial interest in relation to hybridization. 

 In a large number of insects, chiefly Hemiptera, Montgomery 

 has found pairs of chromosomes, which he terms heterochromo- 

 somes, much smaller or much larger than the others and these 

 may be followed through mitosis from one nucleus to another. 

 The heterochromosomes of each pair are known to unite with 

 one another during synapsis to form the bivalent chromosomes 

 of the reduction mitoses and they then become separated, each 



