STKUCTUKE OF ORGANISMS. 27 



may be so small as scarcely to be visible with the high 

 powers of the microscope, or as large as the yolk of an 

 egg, for instance, the greater portion of which is, however, 

 made up of yolk granules. 



If living organisms did not reproduce, they would 

 sooner or later be extinguished, if not by natural at all 

 events by accidental death. Therefore only those 

 creatures survive which can multiply, and indeed multi- 

 ply rapidly by the separation of some portion of their 

 substance capable of growing into a new individual. 

 This may give rise to a vegetative or asexual mode of 

 reproduction by fission, or by the formation of special 

 reproductive cells, or spores, or cellular bodies (buds). 

 Mention has already been made above of asexual re- 

 production in connection wit!/ growth and regeneration. 

 Propagation by shoots developed on runners or on tubers 

 is common among the higher plants, and gemmation or 

 bud-formation is a widely distributed mode of reproduc- 

 tion among animals. It is found not only among lowly 

 organised forms like the zoophytes, and polyps (Ccelen- 

 terata), and sponges (Porifera) ; but even in such highly 

 differentiated groups as the marine bristle- worms (Chaeto- 

 poda), sea-mats (Polyzoa), and sea-squirts (Tunicata). 

 But while in the Bacteria asexual reproduction by fission 

 and by spores is the only known method of propagation, 

 in all other groups of plants or animals sexual reproduc- 

 tion occurs, though the asexual method may also be 

 retained. 



The typical sexual reproduction of multicellular plants 

 and animals takes place by means of special cells of two 

 kinds set apart for the purpose, and called the germ-cells 

 or gametes. Each gamete of one kind by fusion with a 

 gamete of the other kind in the process of fertilisation 

 gives rise to a single cell, the zygote, which grows into a 

 new individual. The two kinds of cells have become 

 differentiated along divergent lines, being adapted to the 

 particular functions they have to perform. One, the 



