4IO PHYSIOLOGY OF THE INVERTEBRATA. 



The ovary in Hydra is a small group of ectodermal or 

 interstitial cells. One cell, however, lying in the centre of 

 the group is larger and clearer than the rest; from this 

 central cell the naked amoeboid ovule is produced. 



In Hydra, as already stated, the sexes are united in the 

 same individual ; but in other Codenterata they are distinct ; 

 " with the colonial polyps the sexes are separate, and each 

 colony may be composed of individuals which are androgynous, 

 or of one sex alone. Some species are sexless, and remain 

 so ; but they produce by gemmation individuals of a particular 

 character, which have sexual organs." l 



In Hydra when gemmation takes place there is ultimately 

 a complete separation of the buds, but in some Codenterata, 

 there is gemmation without separation of the young; this 

 occurs, for example, among the Coralligena. 



Concerning the development of Hydra, the following is an 

 outline of the process : After the ovule or egg-cell escapes 

 from the ovary it is fecundated by spermatozoa, which are 

 discharged from the testes into the surrounding water. There 

 is no act of copulation. As the result of fecundation the 

 naked egg-cell acquires a cell-wall, and segmentation of its 

 mass follows; that is, a morula or blastosphere is formed. 

 After this a chitinous shell is elaborated which envelopes the 

 embryo. The embryonic cells fuse together, giving the embryo 

 the appearance of an unsegmented egg-cell. In the centre 

 of this mass a small cavity (the beginning of the body cavity) 

 is produced. This gradually widens and lengthens so that 

 the embryo becomes a closed sac. After several weeks the 

 above-mentioned shell is ruptured, and the hollow germ 

 escapes enveloped in a thin membra.ne. The protoplasmic 



lost in a few miiintes. Spermatozoa in the seminal reservoir of the female 

 bee retain their powers for several years. In mammals the seminal 

 elements retain their powers of fertilisation for some time in the genital 

 passages of the female ; in the female rabbit Balbiani found them twenty- 

 four hours after coition ; and Drs. E. van Beneden, Benecke, Eimer, and 

 Fries have observed spermatozoa in the uterus of bats for several months. 



