THE FIRST PRINCIPLES OF HEREDITY 



assimilates it. In this way the cell adds to its substance ; 

 that is, it grows. When it has attained a certain size, 

 which varies for different cells (the average size of a cell is 

 always microscopical, about J^ to ^4^ inch or less), the 

 cell divides into two daughter-cells, each with its own 

 nucleus and cell-body — a process which will be described 

 later on. We have thus, as the outcome of the growth of 

 the cell, reproduction in its simplest form. 



The lowest organisms, as Amoebas and Infusorians among 

 animals, or Bacteria and Yeast among plants, consist only 



of a single cell. They are 

 called Protista — viz., Proto- 

 zoa, if they belong to the 

 animal kingdom, and Proto- 

 phyta if belonging to the 

 plants. In the next stage 

 we find an aggregation of 

 cells, all more or less alike 

 structure, forming cell- 



n- 



pr* 



cw 



m 



Fig. I. — A Cell. 



{From H. W. Conn, " The Story 

 of Life's Mechanism.") 



cw, the cell wall ; pr, the cell 

 substance or protoplasm ; 

 w, the nucleus. 



colonies, as the Algae (plants) 

 or Volvocinese (animals). 

 Gradually differentiation 

 arises among the cells ; an 

 inner and outer layer is 

 formed, each assuming dif- 

 ferent structure and function. 

 As we ascend higher through the classes of animal and 

 plant species further differences ensue ; tissues and organs 

 are formed for specific purposes with specialized anatomical 

 structure, until we reach the highest stage in the mammals, 

 and finally in man. All organisms composed of many cells 

 are called Metazoa and Metaphyta, in distinction from 

 Protozoa and Protophyta respectively. However great the 

 difference between the lowest and highest organisms may 

 be, there is this one fundamental agreement : they all are 

 made up of cells, variously arranged and variously adapted 

 in structure and function for their specific purpose. 



