170 GENERAL BIOLOGY 



Early development. The cellular structure of the earth- 

 worm is best understood when considered in the light of its 

 origin. The worm, like all other organisms begins life as a 

 single cell. In a broad sense, two processes make up its 

 physical career; cell multiplication, and cell differentia- 

 tion. The former necessarily precedes; the latter pre- 

 dominates during the later stages of development, but both 

 take place concurrently throughout life in parts of the body. 



The egg (fig. 106 e] when fertilized is potentially a new 

 worm. Whatever characteristics are to appear in the adult 

 are already inherent in it. It is isolated from further 

 parental influence. It develops of itself. 



FIG. 107. Diagrams of the development of the earth- 

 worm, early a, b, c, 2-cell, 4-cell, and 8-cell stages, 

 respectively; d, e, f, sections of later stages showing 

 gastrulation. and the formation of the mesoderm. 

 (after Wilson) . 



There is nothing of the worm structure visible in it ; we 

 can see in it only the usual parts of a normal undifferentiated 

 cell a nucleus, cytoplasm, and inclusions, within the cell 

 wall. Moreover, there is nothing suggestive of a worm in 

 the earlier of the series of remarkable changes through 

 which it passes in development (fig. 107). It divides into 

 two cells, the two divide into four, the four into eight, the 

 eight into sixteen, etc., by a regular and nearly equal divi- 

 sion. This process is common to all animals above the pro- 

 tozoans, and is called cleavage or segmentation. It results 



