n COELENTERATA 29 



the form of the parent to which it is still attached. Sooner 

 or later it becomes constricted at the base, and then entirely 

 separated ; and in this simple way a new individual is formed, 

 and enters upon a separate existence. 



This method of reproduction might be compared to the 

 growth of a new plant by means of a runner growing out of 

 the parent-plant ; but just as in the higher plants there is 

 normally another method of reproduction, which depends 

 upon the fusion of two special cells — the sperm or generative cell 

 from the pollen grain, and the egg cell in the ovule within the 

 ovary — so here we find another method of reproduction, in 

 which, similarly, an egg cell is formed inside an ovary and 

 is fertilised by a sperm cell. 



In both cases the fusion of the sperm with the egg cell is 

 known as Fertilisation, and the result is to stimulate the egg 

 cell to further growth and division. In the plant, this results 

 ill the formation first of the seed, and then from the seed 

 of the new, young plant. In the animal, the fertilised egg 

 develops directly into the new individual, without a " seed " 

 stage intervening. In the Hydra, the structures are all much 

 simpler than in the higher plants and animals. 



Sexual When this method of reproduction is about to 



Reprodttc- take place, little swellings are to be seen on the 



tion. surface of the body of the Hydra (Fig. 11). Each 

 of these swellings consists merely of an up-pushing of the 

 superficial skin cells, due to the rapid multiplication of the 

 " packing cells " at one spot. The swellings nearer the base 

 of the body become larger than the others ; these are the 

 ovaries, and inside each, one cell enlarges at the expense of the 

 others, engulfing and digesting its companion cells, much as 

 an Amoeba engulfs and digests its fopd ; finally, the single 

 large cell which remains, draws in its irregular processes, 

 and after certain nuclear changes, it becomes the ripe egg 

 cell or ovum, ready for fertilisation, without which stimulus 

 it can now develop no further. 



Meanwhile, the smaller swellings nearer the upper end of 

 the Hydra have become spermaries, for in these each of the 

 " packing cells " becomes a minute sperm cell, not more than 

 ■j^^ of an inch in length. 



These sperm cells are in appearance very unlike the round 

 passive sperms or " generative cells " found in the pollen-tube 



