REPRODUCTION 191 



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. 



In vegetables it cannot be shown that the germinal 

 cells differ essentially from the somatic cells. They 

 appear only when the reproductive process is anticipated, 

 attain to the necessary degree of specialization in a few 

 generations, are characterized by a preparation for 

 fertilization to all intents and purposes identical with 

 that of the animal cells, and then having been fertilized 

 by conjugation with another specially adapted cell, lose 

 the reproductive quality and become vegetative in 

 character once more. Thus, in vegetables the repro- 

 ductive activity may be said to pervade the cells gener- 

 ally, while in animals it is more and more restricted to 

 the few cells comprising the germ plasm. 



