VITALITY 



between the gametes of many species distributed amongst all groups 

 of Protozoa, and here, obviously, the attempt to apply any defini- 

 tion of sex fails completely. Yet such fertilizations are as" fruitful 

 and as important for the species as are those in which gametic 

 differences are well-marked. 



FIR5T MATURATION DIVISION OF MICRONUCLEUS 



SECOND AND THIRD 

 DIVISION OF MICR0NUCLEU5 



k \ m&\ Mm 



THREE SOMATIC DIVISIONS OF FERTILIZED NUCLEUS 



FERTILIZATION 



TWO CONSECUTIVE DIVISIONS 

 GIVING FOUR NORMAL CELLS 



Fig. 139.— Paramecium caudatum. Diagram of the fertilization processes. 

 (After Calkins.) 



There are two fundamental biological problems associated with 

 the formation and fusion of gametes. These are: (1) The expla- 

 nation of the origin of gametic differences, and (2) explanation of 

 the phenomenon of attraction of gametes followed by their tem- 

 18 



