CHAPTER 23 



DEVELOPMENT OF THE INDIVIDUAL 



So far we have seen how cells duplicate 

 themselves, those cells in the body tissues 

 as well as those special cells, the germ cells. 

 We shall now follow the course of events 

 that takes place when the germ cells unite 

 to form a zygote which then undergoes mil- 

 lions of mitoses directed specifically toward 

 the formation of a new organism, different 

 from any other on earth. This subject, em- 

 bryology, is one of the most fascinating, 

 and yet in its fundamental aspects one of 

 the least understood, of any fields of biol- 

 ogy. To realize the magnitude of the prob- 

 lem, consider how svich a tiny object as 



a fertilized egg can carry within itself the 

 potentialities of producing a living organ- 

 ism as complex as man. 



The above question has stimulated specu- 

 lation among thinking men for many cen- 

 turies. Aristotle, for example, believed that 

 the male element or semen was the seed 

 that save rise to the new individual. Indeed 

 the word semen means seed. The female 

 was thought of as the earth ( Mother Earth ) 

 in which the seed was nourished so that it 

 might grow. This was a natural deduction, 

 since it was known that castrated animals 

 were sterile; without semen there was no 



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