46 
SPERMATOGENESIS OF NORMAL 
and equivalent to two of the simple (univalent) type. In sperm¬ 
atogenesis this change generally comes about in the primary 
spermatocyte. Now, in hybrids it may be supposed that in the 
ordinary cells of the body, the chromosomes from the paternal 
and the maternal species lie side by side and carry on the cus¬ 
tomary functions of the cells but when it comes to an actual 
fusion of chromosomes to form the bivalent type necessary for 
reduction, the incompatibility of the two different plasmas 
renders the union incomplete or prevents it entirely. That the 
ordinary somatic cells of hybrids, either plants or animals, are 
under the influence of two distinct tendencies is well shown in 
the decided mosaic-like structures which frequently occur, the 
classic figure being to liken hybrids to warp and woof. To cite 
but one example, Macfarlane^ found that a hybrid of the goose¬ 
berry and black currant instead of being a strictly intermediate 
type, really possessed side, by side, organs characteristic of each 
parent; the leaves bore both the shield-shaped, oil-secreting 
hairs of the currant and the simple hairs of the, gooseberry, 
though each hair was but half the size of the parent type. We 
may infer then that in Hybrid pigeons the univalent chromo¬ 
somes from each of the. parents may lie side by side in the ordin¬ 
ary cells of the body and divide normally, but when it comes to 
the period of fusion in the germ-cell, they will not unite to form 
the bivalent type or else they unite incompletely. The result is 
that in the primary spermatocyte instead of one spindle bearing 
eight bivalent chromosomes, a multipolar spindle, or not infre¬ 
quently two separate spindles, bearing two groups of univalent 
chfomosomes may appear. In cases where both large and 
small chromosomes are seen, it is necessary to suppose that a 
loose union has occured in some, chromosomes. Whether the 
bivalent chromosomes formed under such conditions consist of 
chromatin from only one parent, or whether both parents are 
represented, there, is no means of determining. The unequal 
divisions of the bivalent chromosomes of hybrids indicate that 
such chromosomes have in some way been rendered very un¬ 
stable. 
In chromatin we have a substance which from all we know 
1. Swingle, W. J. and Webber, H. J: Hybrids and Their Utilization In 
Plant Breeding.—Yearbook, Dept, of Agric., 1897. 
