CHAPTER 14 

 REPRODUCTION 



A new animal or plant comes into existence only by the transfor- 

 mation of some part of a previously existing organism. While repro- 

 duction must have been understood for man and his domesticated animals 

 from time immemorial, it is not so long since it was popularly believed 

 thei-e were other ways whereby new individuals could arise. Among the 

 ancient Greeks it was common belief that leaves could be converted into 

 fish or birds, mud into frogs, dead flesh into bees. In the Middle Ages 

 barnacles were thought to be transmuted fruit of a tree, and to give rise 

 in turn to geese. As these notions were abandoned, the idea was trans- 

 ferred to the smaller organisms which improved microscopes were 

 revealing. It was only comparatively recently that the view that 

 bacteria arose de novo from nonliving matter was given up. The sup- 

 posed origin of living things from nonliving matter was called abiogenesis 

 or spontaneous generation. While in the evolution of life there must 

 once have been a beginning of the living out of the lifeless, it is not 

 likely that such changes are happening now. Certainly there is no pro- 

 duction, from nonliving substance, of new individuals belonging to 

 recognized present-day species of animals or plants. 



Increase in numbers of individuals, or replacement of losses, is pro- 

 vided for by a variety of reproductive methods which fall into two 

 general categories, namely, sexual and asexual reproduction. Sexual 

 reproduction as a rule involves two parents and the union of two germ 

 cells, or of two cells of some kind, or of two nuclei of different cells. 

 Asexual or nonsexual reproduction includes all forms of reproduction 

 not involving germ cells or any of the unions just named. 



Sexual Reproduction. — Sexual reproduction is a well-nigh universal 

 method of reproduction. It is employed by representatives of every 

 great group of animals and by many of them to the exclusion of the 

 asexual method. It is also used by the plants, except the bacteria. 



In one of its very common forms, sexual reproduction is the union of 

 two cells to form a single cell, the zygote, which by its subsequent divisions 

 produces a new individual (in the metazoa) or a new series of individuals 

 (in the protozoa). Not all cells are capable of uniting in this way, and 

 cells which are capable of this act are called gametes. Certain gametes 

 are relatively large, contain a considerable amount of nutritive material, 



159 



