CHAPTER VIII 



Reproduction and Growth 



IN all animal species except the very lowest, each new 

 individual starts life as an tgg. The tgg is a single cell pro- 

 duced by the female of the species. It cannot develop until 

 it has been entered and fertilized by a single cell called a 

 sperm, or spermatozoon, produced by the male of the species.^ 

 The tgg contains nutritive elements which are drawn upon 

 to build up the new individual. It cannot move. The sperma- 

 tozoon is very much smaller than the tgg. It is equipped with 

 some form of tail, by which it is able to move itself through 

 a liquid medium, just as a tadpole does. It is the active 



Figure 23. RELATIVE SIZES OF EGG AND SPERM IN THE 

 FLOUNDER 



Magnification about 30 diameters. 



^ Exceptions, animals whose eggs develop fartheno genetically (with- 

 out fertilization) have no bearing on this discussion and will not be dealt 

 with here. These are all invertebrates} no vertebrate male has to suffer the 

 indignity of being so ignored. 



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