122 THE SCIENCE OF BIOLOGY 



nucleus " came to be recognized as the fundamental unit, and 

 the inappropriateness of calling such a body a cell was 

 acknowledged (Fig. 11). But the word had become so 

 firmly established in terminology, that it has successfully 



resisted both earlier and 

 later efforts directed toward 

 the substitution of a more 

 suitable term. 



The idea of animal and 

 plant bodies being made of 

 many independent, but at 

 the same time interdepend- 

 ent, units, known as cells, 

 thus came into existence. 

 The simplest organisms 

 were found to be composed 



FiQ.ll. Diagrammatic Figjire of _a Cell. f j } R j general 



Ce, centrosome; chn, chondnosome; . 



chr, chromatin; mb, metaplasmic it was established that Cells, 



body; nu, nucleus; pi, plasmosome; ]^ e the bricks of a wall, 



cy, cytoplasm. . , , . .. . 



make up the whole living 



structure. It was evident, therefore, that cell activities, 

 collectively or individually, formed the basis for the activi- 

 ties of any living organism. All functions of organisms 

 were seen to be cell functions in the ultimate analysis. 

 Such a generalization laid new foundations for biological 

 science. 



As the universality of cells became apparent, their mode 

 of origin was recognized as a problem demanding solution. 

 The formulators of the cell-theory had supposed that cells 

 arose by differentiation from a, formative substance. Within 

 a few years it was proved that cells arise only from pre- 

 existing cells by a process of cell division. Later, it was dis- 

 covered that nucleus arises from nucleus in a similar manner. 

 Following this, it was shown, that ovum and spermatozoon 

 are merely specialized cells. Finally, it was ascertained 

 (1875) that fertilization consists in the union of a single 



