536 HEREDITY 
to appear in the flowers of its progeny. The exact nature of 
the factors or substances which are responsible for the appear- 
ance of characters is not well understood, but it is quite evident 
that they are protoplasmic substances. That they are proto- 
plasmic substances is probably more easily demonstrated in 
the lower than in the higher organisms. In the reproduction of 
simple one-celled plants, like Pleurococcus, the plotoplasm of 
the parent divides, and each half of the parent becomes a new 
individual. The parent thus disappears in the formation of 
new individuals or progeny, which are at first merely segments 
of the parent. The new individuals, as they develop to normal 
size, develop in full the features characteristic of the parent. 
They separate soon after they are formed, develop a lobed chlo- 
roplast, enlarge and thicken their walls with cellulose, and retain 
a globular form. These are the constant characteristics by which 
we know Pleurococcus, despite the fact that there are numerous 
other ways the plant might develop. It might, for example, 
form a filament like that of Spirogyra, develop ribbon-like 
chloroplasts, and enclose itself in a woody wall. The charac- 
ters of Pleurococcus are the results of the way the protoplasm 
works, for the protoplasm forms the chloroplasts, the cell walls, 
and is responsible for the separation and shape of the cells. It 
is obvious that the characters of this simple plant are what 
they are and are constant because the protoplasm has a dispo- 
sition to work only in certain ways and retains this particular 
disposition as it passes from generation to generation. 
In higher plants and animals the parent does not divide and 
each half go to form a new individual, but only a small part of 
the parent, a sperm and an egg, are transmitted to the offspring. 
Hence the sperm and egg must contain all of the protoplasmic 
constituents necessary for producing in the offspring the char- 
acters of the parents. But the sperm consists almost entirely 
of a nucleus, and hence it is believed that the material upon 
which the development of characters depends is within the 
nucleus, and occurs in connection with the chromatin, which, in 
the form of chromosomes, behaves in such a regular way during 
cell division as to suggest a definite relation to heredity. These 
protoplasmic constituents upon which the development of char- 
acters depends, Weismann called determinants, ana some other 
biologists call them genes. 
