6 MONTGOMERY— CELLULAR BASIS OF HEREDITY. [Jan. 15, 



cellular basis. All explanations must remain purely hypothetical 

 until this is done. And here I would call attention, as briefly and 

 concisely as possible, to certain positive results that have been won 

 in the study of the germ cells, and disregard the many fascinating 

 but purely hypothetical views as to the process of heredity. 



II. 



The statement of the problem must be a very broad one. The 

 fertilized egg gradually cleaves into many cells. These progressively 

 arrange themselves into tissues, and these form organs. By con- 

 tinuing cell division, by change of position and infolding of cells, 

 and particularly by a differentiation of the cells as the development 

 proceeds, the adult organism eventuates. Then from the body of 

 this adult comes an egg, and it repeats the whole involved process. 

 Here are two great fundamental problems : the one, why the 

 offspring resembles the parent ? the other, what are the factors of 

 differentiation ? On the answer to these problems depends to great 

 extent the explanation of how variations arise and how they are 

 promulgated, that is, the explanation of descent with modification, 

 broadly called evolution. The very subsidiary question of the 

 determination of sex is necessarily also connected with these prob- 

 lems. And all of these questions are inseparable from the one : 

 How far is the adult preformed or prelocalized in the germ cells ? 



What interests us immediately are the two points : First, has there 

 been empirically determined a particular cellular substance, most 

 intimately connected with the transmission of hereditary growth 

 energies? And second, if such a substance is known, does its 

 behavior during the process of development of the embryo throw 

 any light upon the processes of heredity ? 



III. 



To make the following argument clear, we must call to mind the 

 structure of the mature germ cells and the process of cell division. 



The maternal germ cell, the ovum, appears much like any large, 

 unspecialized cell. We distinguish in it a central rounded body, 

 the nucleus, with its surrounding cytoplasm. In the cytoplasm 

 there is a living substance, the protoplasm proper, and various deu- 

 toplasmic substances, such as yolk, which serve mainly for the 

 nourishment of the cell. The nucleus is more complex. Travers- 



