CHAPTER LIII 

 DEVELOPMENT 



325. Many cells from one cell. Every cell that we have 

 studied originated by the dividing of some preexisting cell. 

 We should therefore suppose that at some time in the past our 



FIG. in. Development of lancelet 



a, the earliest stage of the animal, consisting of a single cell ; this divides into two 

 cells, b ; c, the four-cell stage ; d, eight cells ; e, a hollow ball resulting from successive 

 cell divisions ; f, the same still further advanced ; in g one side of the sphere has begun 

 to cave in ; this process continues until the opposite walls meet, forming a double 

 walled, cup-shaped structure, h 



own bodies were made up of fewer cells than they contain to- 

 day. Then what is the smallest number of cells of which an 

 individual animal may consist ? We know, of course, that there 

 are one-celled plants and one-celled animals. But are there 

 one-celled oak-trees and one-celled elephants ? An examination 

 of the facts of development shows us that every individual, 

 plant or animal, starts life as a single cell. 



If we begin with this one-celled body, we can understand 

 from our earlier studies how it may become a many-celled 

 body (Fig. in). 



274 



