Reproduction and Development. 



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. 



YA. 



B: / . 7 c> x 



Fig. 8. Hydra viridis. 



A. hydra half retracted, with a bud and an ovum attached to the shrunken ovary; B. a* 

 sni.-ul hydra firmly retracted; C, a hydra fully extended. </., bud; /., lout; /i.., hy^ustuuie; 

 win., ovum; ovy., ovary; ., teutacles; is., 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 r 

 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 



