CHAPTER V. 



E:MBRTOLOaY AND EEPRODUCTIO^^. 



The faculty of reproducing from aa egg oi* germ the 

 facsimile of themselves is the most strikiug character- 

 istic of living objects, as distinct from objects not 

 possessed of life. Where an animal or plant consists 

 of one cell only, it is obvious that the growth of a 

 new cell, if perfectly similar to the first one, is repro- 

 duction, so that in such cases it is often, though by 

 no means always, not possible to distinguish a special 

 reproductive process. But in multicellular plants 

 and animals, special cells are told off for the work 

 of reproduction, and modified accordingly, as is the 

 case with every other vital process or function. These 

 cells and their functions may best be understood by 

 referring to the facts observed in certain unicellular 

 organisms. Two unicellular individuals, alike except 

 in size, or nearly so, touch one another and fuse, their 

 soft protoplasmic substance running together ; theii* 

 two nuclei fuse to form one nucleus, and their out- 

 lines become rounded off, so that they form one cell. 

 This process, the union of two similar cells, is called 

 conjugation (Fig. 21) : it is comparable with the repro- 

 ductive process in certain plants, viz., two orders of 



