ONTOGENESIS 141 



and, being scattered by various agencies far and wide, 

 bring about an extensive distribution of the species. 

 The method of propagation just outlined, whether 

 by spores or by buds, by simple or multiple fission, 

 is termed vegetative or asexual reproduction. We 

 may define it as the cutting off from an organism of 

 a single mass of protoplasm (of one or many cells) 

 which independently differentiates into an organism 

 resembling its parent. The greatest diversity in 

 vegetative reproduction is to be observed in the 

 various groups of animals and plants, but in every 

 case it can be explained as a special form of the 

 growth process. 



SEXUAL REPRODUCTION 



In nearly all forms of organic life, reproduction is, 

 complicated by an accompanying phenomenon, the 

 meaning of which is not at all clear. In the bacteria 

 and a few other Protista vegetative reproduction 

 is the only sort known. In all others we find that, 

 either accompanying the vegetative reproduction, 

 or alternating with it, there occurs the production of 

 certain reproductive cells (germ-cells) to which the 

 name gamete is given. Two of these gametes fuse 

 together, either completely or partially, and from the 

 fused cell (called the zygote) a new individual devel- 

 ops, or, more accurately, the zygote transforms by 

 growth and differentiation into a new individual. 

 When the two cells fuse completely into one, in the 

 formation of the zygote, the process is termed ho- 

 logamy. When the fusion involves only the nucleus, 



