REPRODUCTION 203 



Development by gemmation also disappears very early, 

 so that there is nothing in the higher animal organisms 

 that is in the least degree comparable to the multiplication 

 by budding so prevalent among the higher plants. Re- 

 production among animals, therefore, soon narrows itself 

 down to the formation of gametes, produced by sexually 

 different individuals, the conjugation of these gametes 

 and the formation of a zygote or fertilized cell which grows 

 into an embryo, which after certain metamorphoses de- 

 velops into a sexual individual resembling one or the other 

 parent. 



There is an early differentiation of the cells of animals 

 into those known as somatic, of which the body proper 

 consists, and those known as germinal whose office is 

 solely reproductive. The latter are contained in the 

 gonads or sexual organs until the organism to which 

 they belong becomes sexually mature, when they them- 

 selves undergo certain essential changes preparative to 

 the fertilization that initiates the reproductive process. 

 The function of reproduction, among the higher animals, 

 usually develops at the time of maturity or complete 

 physical perfection. There are, however, exceptions, as in ' 

 the case of the axolotl (larva of Amblystoma mexicanum), 

 a batrachian, which is capable of sexual reproduction in 

 the larval stage. 



Another illustration of parthenogenetic development 

 and exception to the customary modes of reproduction 

 in certain insects (Cecidomyia) was discovered by Wagner 

 in 1861. Within the bodies of the larvae of these insects, 

 daughter larvae, exactly like the parent, develop and sub- 

 sequently creep out. These parthenogenetic larvae develop 

 similar broods, and these others, throughout the entire 

 autumn, winter, and spring months. With the last gen- 

 eration, at the beginning of summer, this mode of repro- 

 duction ceases and the larvae pupate, producing both males 

 and females that engage in ordinary sexual reproduction. 

 Such alternation of sexual with parthenogenetic repro- 

 duction Wagner called pcedogenesis. 



