REPRODUCTION. 73 



cells, known as germ-cells, like the eggs of animals and tlie 

 spores of ferns and mosses. Only the germ-cells (which may 

 conveniently be distinguished from those forming the rest of the 

 body, or the somatic cells), escape death, and that only under 

 certain conditions. 



All forms of reproduction fall under one or the other of two 

 heads, viz., Agamogenesis {asexual reproduction) or Gamogenesis 

 {sexual reproduction). In the former case the detached portion 

 (which may be either a single cell or a group of cells) has the 

 power to develop into a new individual without the influence of 

 other living matter. In the latter, the detached portion, in tliis 

 case always a single cell (ovum, ousphere, etc.), is acted u})(>n 

 by a second portion of living matter, likewise a single cell, wliich 

 in most cases has been detached from the body of another in- 

 dividual. The germ is called the y<2m«Z^ germ- cell ; the cell act- 

 ing upon it the "male gerin-cell / and in the sexual process the 

 two fuse together {fertilization, imjjr eg nation) to form a single 

 new cell endowed with the power of developing into a new in- 

 dividual. In some organisms (e.g., the yeast-plant and bacteria) 

 only agamogenesis has been observed ; in others (e. g. , vertebrates) 

 only gamogenesis ; in others still both processes take place as in 

 many higher plants. 



The earthworm is not known to multiply by any natural 

 process of agamogenesis. It possesses in a high degree, however, 

 the closely related power of regeneration / for if a worm be cut 

 transversely into two pieces, the anterior piece will usually make 

 good or rege'iierate the missing portion, while the j^osterior })i(.'ce 

 may regenerate the anterior region. Thus the worm can to a 

 certain limited extent be artificially propagated, like a plant, by 

 cuttings, a process closely related to true agamc jgenesis. -^ Its 

 usual and normal mode of reproduction is by gamogenesis, that 

 is, by the formation of male germ-cells {sj)er)?iatozoa) and female 

 germ-cells (ova). In higher animals the two kinds uf germ- 

 cells are produced by different individuals of opi)osite sex. The 

 earth w^orm on the contrary is hermaphrodite or lisexual; e\ery 



* Many worms nearly related to Lumhricns—e.g:., the genus Duo, and other 

 Naids — spontaneously divide themselves into two parts each of which becomes 

 a perfect animal. This process is true agamogenesis, though obviously closely- 

 related to regeneration. 



