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 (Chcetogaster 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 (Microstomum lineare, a Turbellarian) may 

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

 viduals. 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, 



