REPRODUCTION 5 



sex) meet and fuse to form a single cell, the fertilised 

 ovum. This new individual then undergoes a series of 

 divisions, becoming multicellular, and in its turn throwing 

 off cells which become parents in due course. 



It will now have become evident, that as the single cell 

 from which all the cells forming the body of the multicellular 

 individual are produced is formed from the fusion of two 

 cells, one derived from each parent, every cell entering into 

 the formation of the various tissues of the body probably 

 contains matter originally derived from each parent. As we 

 shall see later, this is actually the case. 



Every multicellular organism begins its existence as a 

 single cell. It is evident, therefore, that until the first 

 division occurs, this single cell itself performs all the func- 

 tions necessary to its own well-being. Indeed for the time 

 this single cell is to all intents and purposes a unicellular 

 individual, and particularly in the case of some plants it 

 leads a comparatively prolonged existence as such. In many 

 of the higher animals, the cells that are eventually going to 

 be thrown off to form new individuals the sexual cells or 

 gametes are differentiated at a very early period, frequently 

 long before birth. These cells live a parasitic existence in 

 the body which has produced them, and in a certain sense 

 can hardly be regarded as forming an integral part of it. 

 They are certainly not necessary to the individual, but they 

 are necessary for the production of new individuals, and are 

 thus essential to the race. 



A very interesting comparison has been drawn with regard 

 to these facts by Biitschli arid Minot, between the life-cycle 

 in a swarm of unicellular animals, and the successive pro- 

 duction of new multicellular individuals. 



According to this theory, the numerous generations of 

 unicellular individuals produced by simple division corre- 

 spond to the cells forming the body or soma of the multi- 

 cellular individual. The number of generations without 

 fertilisation (conjugation) in the unicellular form is, under 

 ordinary circumstances, limited. Upon the occurrence of 



