The evolution of living organisms / by Edwin S. Goodrich.

  • Edwin Stephen Goodrich
Date:
[1912]
Catalogue details

Licence: Public Domain Mark

Credit: The evolution of living organisms / by Edwin S. Goodrich. Source: Wellcome Collection.

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    For the life of a cell both body and nucleus are essen- * tial. An interchange of material takes place between them ; one cannot live without the other. If a cell is divided into halves, only that which contains the nucleus will continue to live, grow, and reproduce. The cell is the smallest known unit of life. In the Bacteria there is no well-defined nucleus, the chromatin being scattered. They probably represent a primitive condition before the typical cell-structure was e o m pie tel y diff erenti a t ed. Among the lowest plants, Protophyta, and the lowest animals. Protozoa , the whole individual consists of a single isolated cell, which consequently has to perform all the functions of life. Such organisms may, nevertheless, become highly differentiated developing special cell- organs—a. mouth for taking in food, motile processes for locomotion, contractile vacuoles for excretion, and so forth. But in the multicellular plants. Metaphyta, and animals. Metazoa, where the cells form tissues and organs, starting from primitive cells capable of performing all the essential processes of life (irritable, metabolic, growing, and reproductive) the cells become as differ¬ entiation advances adept at performing certain functions, but correspondingly incapable of carrying out others. The more they become adapted in special directions, the more they are apt to lose their other powers. So the cells of the alimentary, nervous, muscular, and other tissues of animals can only continue to live in association with each other, supplying each other s wants. More¬ over, the specialised cells of differentiated tissues can usually only produce by division cells like unto them¬ selves, and may lose the power of reproducing at all. Multicellular organisms all start from a cell in a primi¬ tive undifferentiated condition, rich in unspecialised protoplasm, capable of becoming adapted to perform any of the necessary functions of life. Of such are built up the early embryonic stages, and the actively growing tissues of adults. But soon they become burdened with the accumulated products of their own activities, their potentialities are narrowed down, their irritability re-