536 HEREDITY 



to appear in the flowers of its progeny, Tlie exact nature of 

 tlie 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 

 pi'otoplasmic constituents upon which the development of char- 

 acters depends, Weismann called determinants, and some other 

 biologists call them genes. 



