Properties and Activities of Living Protoplasm 95 



constant form characteristic for that kind of crystal. In this case the 

 raw material used in building is essentially the same as the final product, 

 and the increase in size has been by accretion (ak -re' shun) (L. ad, to; 

 cr esc ere, to add or grow). This is to be distinguished from true growth 

 by intussusception (in tus sus -sep' shun) (L. intus, within; suscipere, to 

 make up), in which chemical rearrangements result in growth from 

 within. In nonliving things the method of size increase and the chemi- 

 cal composition are determined by the specific nature of the beginning, 

 raw material, while in true growth the nature of the final product 

 formed is determined by the protoplasm of the organism involved. A 

 cat and dog may be fed the same kind of food, but each animal assimi- 

 lates and grows in a manner unique for it. There are still many un- 

 solved problems in connection with how growth is initiated, how it 

 continues, and also how it ceases. It is not only important to have a 

 tissue or organ grow to be normal, but it is equally essential to have it 

 cease growth when normality has been reached. Such abnormal growth 

 as tumors, cancers, etc., still constitute major problems in this field. 



From what has been said it would seem that a cell dividing by mitosis 

 would always produce two identical cells which eventually would be 

 similar to the original. Frequently, this does happen, as in the forma- 

 tion of similar cells in the diflferent tissues. However, if this always 

 happened, a multicellular man, who develops from an original cell by 

 repeated mitoses, would be composed of cells which would all be alike, 

 because the chromosomes, genes, etc., are supposedly alike. From our 

 studies of tissues it was observed that there are many diflferences in 

 structures and functions in various tissues, which originally all arose 

 from the first cell. How can different cells arise from a common origin? 



If chromosomes and their genes alone determine the structure and 

 functions of cells, then should not all cells in a multicellular organism 

 be alike? It is thought that genes produce enzymes, or act as enzymes, 

 which influence the behavior of cells under certain conditions. Because 

 of variations in external and internal environmental factors (such as 

 foods, enzymes, etc) which can influence groups of cells, or even the 

 opposite ends of a single cell, differently, even the same genes in the vari- 

 ous cells will not express their inherent potentialities in the same way in 

 all cells. In other words, differences in physical and chemical factors 

 are thought to influence even identical genes in such different ways that 

 variations in structures and functions are developed. This is the basis 

 for cellular differentiation (changes in the organization of protoplasm) 

 which results in difTerent types of cells, tissues, etc. After their differen- 



