TYPES OF CCELENTERA HYDRA. 143 



The reproductive organs. (a) From nests of repeatedly-dividing 

 interstitial cells, several (1-20) simple male organs or testes are formed. 

 Each consists merely of a clump of male elements or spermatozoa, 

 bounded by the distended ectoderm. Through this the spermatozoa 

 are extruded at intervals, and one may fertilise the ovum of the Hydra. 

 In other words, self- fertilisation, which is very rare among animals, 

 may occur. The spermatozoon is a motile cell, with a minute cylin- 

 drical " head " consisting of nucleus, a more minute middle piece, and a 

 long thread-like vibratile tail (Fig. 66, i). 



(b] Usually there is but one female organ or ovary, but in H. fusca 



. - ^ 

 ' ' 3 



.-,-.. 



-It.-. 



' VV 



-. 



;. - ',' 



4 



FIG. 66. Development of Hydra. After Hrauer. 



1. sp., spermatozoa. 



2. Amoeboid ovum ; g.v. , germinal vesicle or nucleus ; y.s., yolk 



spherules. 



3. Ovum with lobed envelope (s/i.) around it. 



4. Ovum protruding ; ., the nucleus ; ect., the ruptured ectoderm ; 



end. , the endoderm. 



5. Section of blastosphere Ect., ectoderm ; End., endoderm 



being formed. 



6. Section of larva. Ect., ectoderm; End., endoderm ; g.c., gut 



cavity : sh., ruptured envelopes. 



as many as eight have sometimes been observed. The ovary arises like 

 the testes from a nest of interstitial cells, one of which becomes the 

 ovum. In rare cases there are two ova. The ovum is at first amoeboid 

 and transparent, but, like many other ova, it feeds on its neighbours, 

 loses its amoeboid form, and becomes rich in nutritive material and 

 in pigment. The same process of exploitation is well seen in the 

 oogenesis of Tubularia larynx, a common marine polype. . It illus- 

 trates the struggle for existence among germ cells. 



Development. The ovum of Hydra is the successful central 

 cell in the ovary. It is at first amoeboid, and becomes more and 

 more rich at the expense of. its neighbours. Their remains (perhaps 



