30 INTRODUCTION TO ZOOLOGY chap. 



of .most higher plants, to which, however, they correspond. 

 Here each consists of a small oval head — formed from the 

 nucleus of the cell — and a vibratile tail of protoplasm. By 

 the vibration of these tails the sperms can move rapidly, and 

 in so doing they break down the spermary wall covering 

 them, and swim out into the water, making their way to an 

 ovary. Many of them may approach a single ovary, but only 

 one of them makes its way through the now ruptured ovary 

 wall into the egg cell, fusing with it, and so accomplishing 

 the act of fertilisation. 



The fertilised egg has now the power of dividing, and it 

 does so rapidly, the cells formed all remaining in close contact, 

 in a little solid mass of cells, which is the little embryo Hydra. 

 This secretes round itself a chitinous membrane, falls to the 

 bottom of the water, rests for a time, and then continues its 

 development into a new Hydra like its parent. 



This method of reproduction may be taken as being in 

 essential points similar to that which obtains in all the higher 

 animals, though it is only in the lower forms that we find 

 both ovaries and spermaries functional in the same individual. 

 When this is the case the animal is said to be hermaphrodite,'^ 

 a term which is also used to describe a flower containing 

 both ovary and stamens. 



In comparing this method of sexual reproduction with 

 that in Vorticella (p. 16), it is clear that the sperm cell may 

 be compared to the microgamete and the egg cell to the 

 macrogamete ; but whilst in Vorticella the two gametes are 

 essentially separate complete individuals — one of the two at 

 any rate being capable of further independent life even if 

 fusion does not occur — in Hydra the gametes are merely 

 special cells differentiated in the body, which are incapable of 

 further development unless stimulated by fusion, and which, 

 after such fusion, develop into a new individual completely 

 independent of the parent form. 

 Begenera- Although the multicellular Hydra is one single 



tion. organism, all the cells with their different func- 

 tions together completing the individual, yet its structure is 

 very simple compared with that of the higher animals, and the 

 differentiation of the parte of the body is not very deep-.seated. 

 This is demonstrated by the fact that the organism possesses 



' From the names of the Greek god and goddess, Hermes and Aphrodite, 



