408 



INTRODUCTION TO CYTOLOGY 



Notwithstanding the fact that many changes have been made in its 

 details, Weismann's theory of heredity proved to be of much greater value 

 than his theory of development. Morgan (Morgan et al. 1915, pp. 223- 

 227) points out that Weismann made three contributions to the study of 

 genetics, which may be stated in three propositions: (1) The germ-plasm 



SOMATIC CELLS 



B 



SOMATIC CELLS 



FIG. 159. The behavior of the hereditary units in ontogenesis according to Weismann 

 (A) and the current interpretation (B). In A the determinants in the nucleus (1, 2, 3, 4) 

 are supposed to be distributed differentially to the various somatic cells. In B the genes 

 (1, 2, 3, 4) are distributed equally to every cell, but the cytoplasm is distributed differen- 

 tially. The same genes working upon different cytoplasms produce different results in 

 various somatic cells. (Diagrams and legend from Conklin, 1919-1920.) 



contains independent elements which may be substituted one for another 

 without undergoing change; (2) a segregation of maternal and paternal 

 factors, pair by pair, occurs at one period in the history of the germ cells ; 

 (3) the behavior of the chromosomes is specifically applicable to the 

 problems of heredity. In these principles are found "the basis of our 



