236 REPRODUCTION 



reproduction, which is comparable from one end of the Animal 

 Kingdom to the other, and also in plants. Not only do unicellular 

 animals reproduce by means of cell division and conjugation; but 

 every multicellular animal is, at one stage of its life cycle, circum- 

 scribed within the Hmits of a single cell, the zygote, which is 

 formed by fusion of the ovum and spermatozoon, and from which 

 the many-celled adult animal develops. One of the ultimate 

 problems in Embryology is to discover how a thinking man can 

 arise from a single cell that bears no resemblance to the adult 

 organism. Reproduction, like metaboHsm and irritability, is 

 reducible to cell activities. As a matter of convenience, reproduc- 

 tion has been separated from the process of development, which is 

 described in later chapters, although the two are but different 

 aspects of the reproductive cycle in metazoa. 



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