2 54 COELENTERATA HYDROZOA CHAP. 



before they themselves produce buds, but in H. oligactis the 

 buds often remain attached to the parent after they themselves 

 have formed buds, and thus a small colony is produced. Sexual 

 reproduction usually commences in this country in the summer 

 and autumn, but as the statements of trustworthy authors are 

 conflicting, it is probable that the time of appearance of the 

 sexual organs varies according to the conditions of the environ- 

 ment. 



Individual specimens may be male, female, or hermaphrodite. 

 Nussbaum ^ has published the interesting observation that when 

 the Hydras have been well fed the majority become female, 

 when the food supply has been greatly restricted the majority 

 become male, and when the food-supply is moderate in amount 

 the majority become hermaphrodite. The gonads are simply 

 clusters of sexual cells situated in the ectoderm. There is no 

 evidence, derived from either their structure or their development, 

 to show that they represent reduced medusiform gonophores. 

 The testis produces a number of minute spermatozoa. In the 

 ovary, however, only one large yolk -laden egg-cell reaches 

 maturity by the absorption of the other eggs. The ovum is 

 fertilised while still within the gonad, and undergoes the early 

 stages of its development in that position. With the differentia- 

 tion of an outer layer of cells a chitinous protecting membrane 

 is formed, and the escape from the parent takes place." It seems 

 probable that at this stage, namely, that of a protected embryo, 

 there is often a prolonged period of rest, during which it may 

 be carried by wind and other agencies for long distances without 

 injury. 



The remarkable power that Hydra possesses of recovery from 

 injury and of regenerating lost parts was first pointed out by 

 Trembley in his classical memoir,^ 



A Hydra can be cut into a considerable number of pieces, 

 and each piece, provided both ectoderm and endoderm are, re- 

 presented in it, will give rise by growth and regeneration to a 

 complete zooid. There is, however, a limit of size below which 

 fragments of Hydra will not regenerate, even if they contain 



1 Verh. Ver. Rheinland, xlix. 1893, pp. 13, 14, 40, 41. 



^ For an account of the development and of the chitinous membrane see A. 

 Br.iuer, Zeitschr. f. wiss. Zool. lii. 1891, p. 9. 



^ Trembley, Memoircs pour servir ii VHistoire d'un genre de Polypes d'cau douce, 

 1744. 



