THE CELL-DOCTRINE 123 



spermatozoon with a single ovum, and, more important 

 still, in the union of nucleus with nucleus. The continuity 

 between generations was thus proved to be a continuity of 

 cells. 



The foregoing facts constitute the foundation for the 

 modern concept of development. The material basis of 

 ontogeny, and so of heredity, is cellular. All present theories 

 of differentiation, growth, heredity, and the like, rest upon 

 the broader hypothesis of cellular organization. Like the 

 evolutionary theory, the cell-theory became at once a great 

 unifying generalization. The biologist recognized the cells, 

 not only as units of structure and function in the existing 

 organism, but as a key to the past. The living substance 

 was revealed as a continuous, never-dying stuff, which could 

 be traced back through many cell divisions to egg and 

 spermatozoon, and thence to preceding generations. These 

 facts led to the inference that the protoplasm had persisted 

 through an infinitude of cell divisions since its first cell 

 organization in the remote past. Thus it consists of the 

 mortal body-cells, which constitute the adult bodies exist- 

 ing at any given moment, and of the potentially immortal 

 ova and spermatozoa. The origin and nature of these germ- 

 cells, their union in fertilization, and the processes of cell 

 division and differentiation by which the adult organism 

 arises from the single cell formed by their union, are all parts 

 of the cell-theory as developed during the last fifty years. 

 Cellular phenomena will probably remain an outstanding 

 feature of the developmental process for all time. 



The establishment of a cellular continuity between suc- 

 cessive generations was a final blow to the recurrent theory 

 of spontaneous generation. The results obtained by Spal- 

 lanzani (1777), by Franz Schultze (1836), and later by 

 Schwann and others, who found that life came only from pre- 

 existing life, were for the first time fully explained. It was 

 shown that the living substance was composed exclusively 

 of cells and that it arose solely by division of antecedent 



