EEPEODUCTIVE ORGANS 21 



They are formed by local multiplication of 

 the iaterstitial cells of the ectoderm, giving rise to 

 small conical heaps, which project externally and 

 are covered on their outer surfaces by capsules 

 formed by the modified large ectoderm cells. 



The interstitial cells become directly con- 

 verted into spermatozoa by modification of their 

 nuclei to form the heads, and elongation of their 

 protoplasm to form the long swimming tails. 

 When fully formed they are discharged by rup- 

 ture of the capsule at its most prominent point, 

 i. The ripe spermatozoa. 

 Select a Hydra in which the testes are of large size ; put 

 it in a drop of water on a slide ; cover it ; and, by gentle 

 pressure on the cover-glass, rupture one of the testes, and so 

 dispha/rge the spermatozoa. Examine with a high power. 



The spermatozoa are fihform bodies about 

 3^ inch in length. Each consists of a small 

 oval highly refractive head, containing the 

 nucleus, and a long actively moving thread- 

 like tail. 



b. The ovaries usually develop a Utile later than the 

 testes. In Hydra viridis there is seldom more 

 than a single ovary present at a time, but in the 

 brown species there may be as many as eight. 

 They form, when ripe, spherical projections of the 

 body-wall considerably larger than the testes, and 

 placed as a rule nearer the basal end of the animal. 

 In each ovary only a single ovum is produced. 

 The ovary is formed like the testis by multiplica- 

 tion of the interstitial cells to form a small heap. 

 One of these cells, near the centre of the heap, 

 soon becomes larger than the rest and becomes 

 the ovum. This grows rapidly at the expense of 

 the other cells of the ovary, which form a capsule 

 around it and supply it with nutriment. The 

 ovum is at first amoeboid so as to come into 



