Animal life and intelligence . liated layers whichline the cavities and spaces of the organism. Sometimesthe individual sponge produces only ova; sometimes onlysperms; sometimes both, but at different periods. Thecells which become ova increase in size, are passive, andrich in reserve material elaborated by their cells which become sperms divide again and again, Reproduction and Development. 43 and thus produce minute active bodies, adance with rest-less motion. These opposite tendencies are repeated andemphasized throughout the animal kingdom—ova relativelylarge, passive, and a
Animal life and intelligence . liated layers whichline the cavities and spaces of the organism. Sometimesthe individual sponge produces only ova; sometimes onlysperms; sometimes both, but at different periods. Thecells which become ova increase in size, are passive, andrich in reserve material elaborated by their cells which become sperms divide again and again, Reproduction and Development. 43 and thus produce minute active bodies, adance with rest-less motion. These opposite tendencies are repeated andemphasized throughout the animal kingdom—ova relativelylarge, passive, and accumulative of reserve material;sperms minute, active, and the result of repeated active sperm, when it unites with the ovum, importsinto it a tendency to fission, or cleavage ; but the resultingcells do not part and scatter—they remain associatedtogether, and in mutual union give rise to a new sponge. In the hydra, generally near the foot or base of attach-ment, a rounded swelling often makes its appearance in. Fig. 8.—Hydra viridis. A, hydra half retracted, with a bud and an ovum attached to the shrunken ovary; B, asmall hydra firmly retracted; C, a hydra fully extended. o., bud; /., foot; , hypostome;, ovum; ovy., ovary; t., tentacles; ts., testis. autumn. Within this swelling one central cell increasesenormously at the expense of the others. It becomes anovum. Eventually it bursts through the swelling, butremains attached for a time. Rarely in the same hydra,more frequently in another, one or two swellings may beseen higher up, beneath the circle of tentacles. Withinthese, instead of the single ovum may be seen a swarm ofsperms, minute and highly active. When these are dis-charged, one may fuse with and fertilize an ovum, occa-sionally in the same, but more frequently in anotherindividual, with the result that it develops into a newhydra. Here there are definite organs—an ovary and a 44 Animal Life and Intelligence. testis—producing the ova or
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