AND OF HYBRID PIGEONS. 
55 
asymmetrical distributions of the chromatin in cell division. 
These irregularities are much more infrequent in fertile hybrids. 
5. Sterile hybrids show in addition^ a deformed sperma¬ 
tozoon and often a marked degeneration of the germinal cells. 
6. The irregularities of division are confined for the most 
part to the primary spermatocytes. Likewise it is in these cells 
that the formation of bivalent chromosomes occurs normally. 
In hybrids, it would seem that the conflicting tendencies of the 
two parental plasmas frequently render the union of the single 
chromosomes to form the double, (bivalent) types impossible or 
abnormal. There seems to be an attempt on the part of each 
plasma to assert its individuality. This visible incompatibility 
of the chromosomes from widely different species serves as an 
index to a kindred lack of harmony between the plasmas of more 
nearly related forms, so that even though pseudo-reduction 
does occur and normal division of the bivalent chromosomes 
follows, the identity of the individual species is still retained 
through the segregation of the maternal and paternal chromo¬ 
somes into separate cells, which may be considered “pure” 
germ-cells (containing qualities of only one species). 
7. Union of two cells containing characteristics of the 
same species would occasion a reversion to that species. Union 
of two cells representing each of the two original species would 
yield an offspring of the. mixed type. The latter would pre¬ 
dominate because of the greater probability of such union. 
Besides, through the mixing just indicated, variability may be 
due also in some cases to the not infrequent inequalities in the 
division of individual chromosomes, through which varying 
proportions of the chromatin of each species may appear in 
certain of the mature germ-cells. 
8. Irregular divisions cannot of themselves account en¬ 
tirely for reversion and variations, because double spindles and 
irregularities in the formation of bivalent chromosomes are by 
far too few to equal the percentage of reversions seen in such 
mild crosses as the brown and the white ring dove. One is 
forced to the conclusion expressed above, that the double- 
spindled and multipolar types of cells which occur in hvbrids 
between very divergent forms are but exaggerated images of 
