THE CELL 7 



the chromatin network and is not stained by ordinary 

 reagents. 



8. Growth by cell division. How does growth take 

 place in a living organism? What are the primary 

 factors concerned in the increase in size? What changes 

 in the organism result in the progressive development 

 of the many useful qualities found in the improved types 

 of the domestic animals? How does a. microscopic egg- 

 cell develop into a mature and highly organized animal 

 possessing countless cells of many different forms and 

 exercising various and important functions? A still 

 more fundamental problem, if possible, is, how are the 

 qualities of an individual transmitted from parent to 

 offspring ? What is the physical basis of heredity and 

 what are the elements concerned in a study of inheritance ? 



Many of these questions are answered by a study of 

 the cell and specifically by a study of cell division. The 

 entire tissue structure of the animal body arises by re- 

 peated division from the germ-cell. The germ-cell itself 

 is the result of the division of a cell which formed 

 a part of the body of the parent. Thus it is that the 

 germ substance carrying the hereditary material is 

 separated from the parent body by cell division. The 

 fertilized egg by continued cell division passes on to every 

 cell in the body a portion of its own substance. The 

 process of cell division must therefore be regarded as a 

 great fundamental fact in the growth and development 

 of plants and animals as well as one of the most significant 

 and primary facts in the transmission of qualities from 

 parent to offspring. 



Growth occurs as the result of continued cell division 

 rather than by any material increase in the size of exist- 

 ing cells. 



