itance material," or more correctly the germinal protoplasm, is composed of ultra-microscopical units which have the power of individual growth and division and which are capable of undergoing many combinations and dissociations during the course of development, by which combinations and dissociations they are transformed into the structures of the adult. Various names have been given to such units by different authors; they are the physiological units of Herbert Spencer, the gemmules of Darwin, the plastidules of Elsberg and Haeckel, the pangenes of de Vries, the plasomes of Wiesner, the idioblasts of Hertwig, the biophores and determinants of Weismann.
With the publication of Weismann's work on the germ-plasm in 1892 speculation with regard to these ultra-microscopic units of life and of heredity reached a climax and began to decline, owing to the highly speculative character of the evidence as to the existence, nature and activities of such units. But with the rediscovery of Mendel's principles of heredity the necessity of assuming the existence of inheritance units of some kind once more became evident, and, without attempting to define what such units are or how they behave modern students of heredity invariably accept their existence. They are now called determiners or factors or genes, and are usually thought of as elements or units of the germ cells which condition the characters of the developed organism, and which are in a measure independent of one another; though of course neither they nor any other parts of a cell are really independent in the sense that they can exist apart from one another. They are to be thought of as we think of certain chemical radicals which exist only in combination with other chemical elements in the form of molecules, and yet may preserve their identity in many different combinations.
If there are inheritance units, such as determiners or genes, as practically all students of heredity maintain, they must be contained in the germ cells, and it becomes one of the fundamental problems of biology to find out where and what these units are. But whether we assume the existence of these units or not we know that the germ cells are exceedingly complex, that they contain many visible units such as chromosomes, chromomeres, plastosomes and microsomes, and that with every great improvement in the microscope and in microscopical technique other structures are made visible which were invisible before, and whether the particular hypothetical units just named are present or not seems to be a matter of no great importance, seeing that, so far as the analysis of the microscope is able to go, there are in all protoplasm differentiated units which are combined into a system—in short, there is organization.
5. Heredity and Development
The germ cells are individual entities and after the fertilization of the egg the new individual thus formed remains distinct from every other individual. Furthermore, from its earliest to its latest stage of
