14 MANUAL OF ANIMAL BIOLOGY 



unique method, known as intussusception, which is characterized 

 by the intercalation of new material among the old. It is quite 

 unlike the process of increase in size by accretion in which the 

 new material is deposited externally on the old, and which is 

 exhibited in the inorganic world in crystal formation. 



Attention should now be called to the fact that the size of any 

 specific type of cell is quite definitely limited, so that when a cell 

 has reached a certain size, growth stops. At this point, provided 

 the dominance of the anabolic processes continues, another char- 

 acteristic feature of protoplasm appears in that the cell has the 

 ability to divide into two daughter cells. This constitutes repro- 

 duction, and the result is the formation of two cells each having the 

 general characteristics of the parent cell. The inherent relations 

 which, it is now apparent, exist between anabolism, growth, and 

 reproduction will be emphasized later in the study of Amoeba 

 and other Protozoa. 



The reproduction of a cell always takes place following a compli- 

 cated process known as mitotic cell division, or mitosis, which 

 involves profound nuclear changes. These result in the correct 

 quantitative and qualitative division of the chromatin material in 

 the nucleus of the dividing cell, and its equal distribution to the 

 two daughter cells. Since, as noted previously, the chromatin is 

 the chief vehicle for the transmission of hereditary characters, 

 the necessity for an accurate division of the material is evident. 

 (W. pp. 239-243.) 



4. Differentiation 



It has been noted above that organisms consist either of one 

 cell or of many cells. Attention should be called at this point to 

 the additional fact that all organisms from the lowest to the highest, 

 including Man, begin life as a single cell — they are originally 

 unicellular — and it is by the continued growth and repeated 

 division of the original cell into two, four, eight, and finally into 

 millions and millions of cells, which remain permanently attached, 

 that the adult organisms are built up. This brings in another 

 important factor, namely, differentiation, which means that 

 groups of cells become structurally modified, or differentiated, to 

 perform a certain task, or function, for the organism as a whole so 

 that in the higher multicellular organisms, or metazoa, as they are 

 termed, there is a division of labor between different kinds of 



