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THE SCIENCE OF BIOLOGY 



cells. The fact that living organisms arise only from parent 

 organisms was explained, when the bodies of animals and 

 plants were shown to consist of living cells which arose by 

 cell division. The inherent falsity of spontaneous generation 

 was thus shown as the cell-theory became extended to the 

 entire field of development. 



m 



m 



FIG. 12. Development of the Frog. Showing probable relations of axes of egg 

 to axes of adult body. A, the egg or ovum at time of fertilization by 

 spermatozoon (s); B, entrance of sperm; C, differentiation of certain 

 portions of adult body (c, en, n, and m), and approach of male and female 

 nuclei; D, two cell stage; E, eight cell stage, F, early blastula; G, early 



