HEREDITY 269 



interaction of the genes in the chromosomes with the cytoplasm 

 and other environmental factors. The genes of the germ cells, 

 because they are shielded in some way from these influences, 

 fail to enter into differentiating processes and thus retain their 

 full potencies as reproductive cells. It is true, of course, that in 

 many animals, notably Coelenterata and Platyhelminthes, the 

 evidence of a germ track from embryology is very unsatisfactory. 

 And it is also true that in plants germ cells may be produced 

 from meristematic cells, which are embryonic plant cells, also 

 present in growing regions of mature plants. However, if the 

 general thesis is true that the genes are the important hereditary 

 material, and that they are located in the chromosomes, it 

 might be maintained that even in these exceptional cases — since 

 every cell in the body possesses the full complement of genes — any 

 cell might become a germ cell. Why this does not ever happen to 

 a definitive somatic cell of higher animals is another matter; but 

 it is probably due to the irreversible nature of the differentiation 

 accompanying normal development, an irreversibility that seems 

 to increase with ascent in the animal scale. 



Mendelian Heredity. — In 1866, Gregor Mendel, an Augus- 

 tinian monk, published the result of experiments with the common 

 garden pea conducted in the monastery at Briinn in what was 

 then Austria. His work remained practically unnoticed until 

 1900 when conclusions similar to his were reached independently 

 by three men, De Vries, Correns, and Tschermak. For this 

 reason Mendel's work does not figure in the literature until about 

 thirty-five years after it was published. His work is important 

 because it demonstrated experimentally the fundamental laws 

 governing the distribution of hereditary characters in offspring, 

 and because it complements in an extraordinary way the chromo- 

 some theory of heredity, which was evolved independently of 

 Mendel's work. The chromosome theory deals with the internal 

 mechanism of heredity, while Mendel's work showed the exter- 

 nally visible results of the operation of this mechanism. A 

 definite step in the direction of a more complete understanding 

 of the working of the mechanism of heredity was made when it 

 was realized that the results of Mendel's experiments and 

 many others since can be readily explained by assuming that the 

 genes for so-called Mendelian characters are located in the 

 chromosomes. 



