TEE CELLULAR BASIS OF EEREDITY 109 



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 combina- 

 tions 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 Hert- 

 wig, 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 inher- 

 itance 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 rad- 

 icals which exist only in combination with other chemical elements in 

 the form of molecules, and yet may preserve their identity in many dif- 

 ferent combinations. 



If there are inheritance units, such as determiners or genes, as prac- 

 tically 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 tech- 

 nique 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. Eeredity 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 



