42 Animal Life and Intelligence. 



by budding. The cut stump of the amputated tentacle of 

 the hydra or the snail buds forth a new organ. But in the 

 hydra, during the summer months, under normal circum- 

 stances, a bud may make its appearance and give rise to 

 a new individual, which will become detached from the 

 parent, to lead a separate existence. In other organisms 

 allied to the hydra the buds may remain in attachment, 

 and a colony will result. This, too, is the result of budding 

 in many of the sponges. In some worms, too, budding 

 may occur. In the fresh-water worm {Chcstogaster limncei) 

 the animal, as we ordinarily see it, is a train of individuals,, 

 one budded off behind the other — the first fully developed, 

 those behind it in various stages of development. The 

 individuals finally separate by transverse division. Another 

 more lowly worm {Microstomuni lineare, a Turbellarian) may 

 bud off in similar fashion a chain of ten or fifteen indi- 

 \iduals. In these cases budding is not far removed from 

 fission. 



Now, in the case of reproduction by budding, as in the 

 hydra, a new individual is produced from some group of cells 

 in the parent organism. From this it is but a step — a step, 

 however, of the utmost importance — to the production of 

 a new individual from a single cell from the tissues of the 

 parental organism. Such a reproductive cell is called an 

 egg-cell, or ovum. In the great majority of cases, to enable 

 the ovum to develop into a new individual, it is necessary 

 that the egg-cell should conjugate or fuse with a minute,, 

 active sperm-cell, generally derived from a different parent. 

 This process of fusion of germinal cells is called fertiliza- 

 tion (see Fig. 5, p. 13). 



In sponges, the cells which become ova or sperms lie 

 scattered in the mid-layer between the ciliated layers which 

 line the cavities and spaces of the organism. Sometimes 

 the individual sponge produces only ova ; sometimes only 

 sperms ; sometimes both, but at different periods. The 

 cells which become ova increase in size, are passive, and 

 rich in reserve material ' elaborated by their protoplasm.. 

 The cells which become sperms divide again and again,. 



