REPRODUCTION AND DEVELOPMENT 251 



which results when two gametes or sex cells unite. This fusion of 

 two specialized sex cells occurs among the Protozoa, for example 

 in the life cycle of Sporozoa, as previously described (p. 75). In 

 animals the two uniting cells are different in appearance and nature 

 (Fig. 133). The ovum, formed in a female organ, is the passive, 

 quiescent member that contains the food materials and is the site of 

 development of the new individual. The other member that enters 

 into the unity, the spermatozoon, is usually active and relatively 

 smaller, consisting largely of nuclear material. An individual or 

 organ which produces this type of gamete is known as a male. 

 In some of the lower plants the two sex cells are not distinguishable, 

 a characteristic which is known as isogamy. Distinguishable differ- 

 ences between the two types of gametes are the rule, however, a 

 characteristic known as heterogamy. With few exceptions (some 

 Protozoa) all animals are heterogamous. 



The union of the two gametes is called fertilization (p. 193). 

 Its functions are several. In the first place, the union of the two 

 nuclei derived from different parents provides a mechanism of 

 inheritance from both, a subject which will be discussed in more 

 detail farther on. Secondly, fertilization in some manner provides 

 a stimulus for development of a new individual by the ovum, since, 

 with certain exceptions to be mentioned presently, neither tgg nor 

 sperm ever develops into an adult unless fertilization has occurred. 

 Thirdly, in some forms of animals the completion of maturing of 

 the egg is not effected until after the sperm has entered. The essen- 

 tial events which follow fertilization will be discussed farther on. 



Metagenesis. At this point it is desirable to discuss those forms 

 which in their life cycles regularly exhibit an alternation in repro- 

 ductive method, sexual and asexual methods succeeding each other. 

 This character, metagenesis, occurs among both plants and animals. 

 As it is impossible to treat of biological phenomena in one type of 

 animal without reference to other types, it is similarly impossible 

 to consider Animal Biology adequately without examining also 



