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 r1 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 re/atively 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 11 chromosomes and two 12. No accessory chromosome 
appears in the mitoses of oógenesis 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 
