Page:Popular Science Monthly Volume 61.djvu/43

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37
THE PHYSICAL BASIS OF HEREDITY.

the parents, a fact of which advantage is taken to preserve the peculiar characters of our varieties of fruit, all of which are perpetuated by grafting, or by means of runners.

In some worms the power of developing a new individual if part of the old one has been lost has been modified so that the lost parts are reformed before they are really lost. A given limited part of the middle of the body has the habit of forming a new head for the part behind, and a new tail for the part in front. A string of individuals is formed in this way joined tandem. These separate after some time and each new individual repeats the process.

PSM V61 D043 Hydra and starfish reproduction.png
Fig. 5. Fig 6
Fig. 5. Hydras: the One at c with Buds; the One at d with Sex-Organs. From Leuckart.
Fig. 6. An Arm of a Starfish reproducing Four Lost Arms.

These methods of forming new individuals are occasional. Each method is restricted to some limited group of species. In practically all animals in which these occasional methods of reproduction occur, they alternate with sexual reproduction. In the great majority of animals sexual reproduction is the only means of transmitting characters to a new generation.

By sexual reproduction we understand the development of a new individual from a single cell which is usually produced by the fusion of two cells.

Just a word as to what we mean by a cell. The word has a penitentiary flavor that may be misleading. A cell is a mass of protoplasm enclosing a differentiated portion or nucleus. The nucleus contains, among other things, during certain phases of cell life a definite number of thread-like bodies called chromosomes. The number of chromosomes differs in different animals, but is always the same in the different cells of the same animal.