MULTIPLICATION OF CELLS 7 



the special work of these organs and tissues, 1 they under- 

 go marked changes in size, shape and behavior. In our 

 study, we are not concerned with their rapid and perplex- 

 ing changes, but with the cells as finally completed. It 

 will be sufficient for us to realize that these changes in the 

 case of a human being are fairly well established within 

 two or three months of the single cell stage. The re- 



FIG. 5. Cleavage or segmentation of an ovum, showing successive division of the 

 germ cell (a) inlo two (6), four (c), and eight (d). Later stages are shown at c 

 and f. The first four figures are diagrammatic : e and / are after Hatschek's 

 figures of the development of a very siinpie vertebrate (Amphioxus). 



maining six or seven months before birth and eigh- 

 teen or nineteen years of growth and development after 

 birth, are devoted to the enlargement and perfection 

 of the organs and tissues 2 already formed in miniature. 



1 Specialization. 



2 The term tissue is applied to such parts of the body as are 

 made up of similar cells, as, for example, muscular tissue and 

 nerve tissue. A tissue may do a special work, in which case it is 

 also an organ. 



