HOW CHARACTERS ARE TRANSMITTED 127 
ovule with its pollen nucleus would develop but a single kerncl, 
and the operation described must be repeated for every one of 
the thousand or more kernels of the car, each of which for 
present purposes is a distinct individual. 
The same is true for each grain of wheat, though in this case 
the ovule and the pollen are produced in the same flower and 
close together under the scale or chaff. So the process could 
be traced for every seed of all species, for each is a new indi- 
vidual. Among animals, also, but two nuclei are involved for 
each new individual whether as small as the cricket or as large 
as the elephant. 
Little enough is known of the essential constitution of these 
remarkable bits of living matter called nuclei, but that little is 
too much to discuss exhaustively here. It is enough for present 
purposes to call attention to the wonderful fact that these two 
bits of matter, too small to be seen and studied with the naked 
eye, carry with them all the characters of the race; moreover, 
as they constitute the only material transmitted from parent to 
offspring, they are the only vehicles of transmission. Other 
nuclei from other parts of the body can repeatedly divide, absorb- 
ing food as they do so, constituting growth, but these nuclei 
from the reproductive cells, excepting in certain lower species, 
do not grow till after union with others from the opposite sex. 
Chromosomes. The nucleus of the animal or plant cell is 
something more than a formless bit of matter endowed with 
life. If the nuclei of several species be stained and examined 
under a high-power microscope, each will be found to contain 
a definite number of rods, rings, or other bodies, always the 
same in all the cells of all the individuals of the same species, 
but differing in different species. These are called chromosomes. 
Another peculiarity about the chromosomes is that for all 
species that propagate bisexually the number is even; thus in 
mouse, trout, and lily it is 24; in ox, guinea pig, onion, and 
1 Tt may be conveniently pursued further in " Principles of Breeding,” chaps. 
vii and viii. 
