Reproduction and Development. 



43 



and thus produce minute active bodies, adance with rest- 

 less motion. These opposite tendencies are repeated and 

 emphasized throughout the animal kingdom — ova relatively 

 large, passive, and accumulative of reserve material ; 

 sperms minute, active, and the result of repeated fission. 

 The active sperm, when it unites with the ovum, imports 

 into it a tendency to fission, or cleavage ; but the resulting 

 cells do not part and scatter — they remain associated 

 together, 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, a 

 small hydra firmly retracted; C, a hydra fully extended. o., bud; /., foot; h.s., hypostome; 

 own.., ovum; ovy., ovary; t., tentacles; ts., testis. 



autumn. Within this swelling one central cell increases 

 enormously at the expense of the others. It becomes an 

 ovum. Eventually it bursts through the swelling, but 

 remains attached for a time. Rarely in the same hydra, 

 more frequently in another, one or two swellings may be 

 seen higher up, beneath the circle of tentacles. Within 

 these, instead of the single ovum may be seen a swarm of 

 sperms, 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 another 

 individual, with the result that it develops into a new 

 hydra. Here there are definite organs — an ovary and a 



