i YT o ” o 
GENETICS AND EVOLUTION 645 
of hybrids there occurs a separation of the determiners of con- 
trasting characters resulting in the production of different kinds 
of germ cells, each of which is pure in respect to any given char- 
acter. ‘This principle was later called “the ‘Splitting | of hybrids” 
by DeVries. 
Probably the most outstanding principle in the Mendelian 
Law of heredity is the fact that the heritable characteristics of 
organisms are independent one of another and may be inherited 
_independently, The determining substances of these character- 
istics must, accordingly, occur in the germ cells as independent, 
self perpetuating particles. Such substances are called genes, or 
factors or unit characters. 
STRUCTURE OF THE CHROMOsOME.—In the resting nucleus the 
nuclear reticulum consists of chromatin threads. These threads 
as often observed are longitudinally double. Each unit of the 
double thread appears as a chain of beads and consists of a 
linear series of granules called the chromomeres and these in turn of 
one or more.smaller particles called chromioles. ‘The chromomeres 
lie opposite each other forming pairs. The determiners within 
the germ cells are believed to occupy the same arrangement, so 
that chromomeres or their components may represent genes. 
MALE AND FEMALE CHROMOSOMES.—Every sexually produced 
organism is a double being. Every cell of a plant or animal is 
double, one half having been derived from the male sexual cell 
and the other half from the female sexual cell. Each of these 
cells contains two sets of chromosomes, one set from the sperma- 
tozoon and one set from the egg. ‘These two sets of chromosomes 
contain the genes derived from both parents. 
During the maturation of the germ cells the chromosomes 
become unravelled. Each paternal chromosome is matched 
by a homologous chromosome of maternal origin. During 
maturation homologous chromosomes and also their chromo- 
meres lie side by side in pairs. At the time of reduction of the 
chromosomes each pair of chromosomes (maternal and paternal) 
appear as two parallel strings of beads, the beads representing 
chromomeres and containing the genes. 
DisTRIBUTION OF CHROMOSOMES DURING MATURATION AND 
FERTILIZATION.—Maturation involves the reduction in the 
