HYDRA 65 



The latter arise, as has been noted, from the interstitial cells of 

 the ectoderm. By repeated divisions of the latter a very large 

 number of immature sperm cells are formed. These undergo 

 radical structural modifications, finally resulting in the formation 

 of mature, free-swimming sperm which break forth from the gonad. 

 They swim about freely in the water, and when one comes in 

 contact with a mature unfertilized egg, it penetrates the egg mem- 

 branes, thus permitting the permanent union of the male nucleus 

 with the female nucleus, that is, fertilization, to occur. (W. f. 

 57, B.) 



The ovaries of Hydra also develop as swellings in the ectoderm 

 of the body wall, but these structures are located farther from the 

 tentacles than are the testes. The eggs which are formed in the 

 ovaries likewise develop from the interstitial cells. The process, 

 however, is different from that which takes place in the formation 

 of the sperm. In the first place there is usually only one egg formed 

 in an ovary, whereas there are enormous numbers of sperm formed 

 in a testis. In the early stages, an ovary of Hydra contains a 

 large number of potential egg cells which have developed from 

 the interstitial cells, but only one of these finally forms a mature 

 egg cell, while the others aid in its nourishment. Some of the sister 

 cells are actually engulfed by temporary pseudopodia which de- 

 velop on the egg cell, and the latter becomes very large as a result 

 of the nourishment supplied by the other cells. The egg, in its 

 general shape and structure, is quite like a typical cell and very 

 different from the atypical sperm cells. The large, mature egg 

 projects through the ectoderm of the Hydra, covered only by a 

 thin egg membrane, and there it awaits the sperm. It remains 

 attached to the ectoderm at this point for some time even after 

 » fertilization has occurred. During later development it breaks 

 away and becomes entirely free from the parent body. The 

 sperm and eggs of any one Hydra do not mature at the same time 

 and, therefore, the eggs are fertilized by sperm released from 

 another individual. (W. f. 57, B.) 



The process of fertilization in the egg of Hydra furnishes not 

 only the method by which, as a result of the fusion of the chroma- 

 tin material, the characteristics from two parents may intermingle, 

 but also the stimulus for cell division. This is also true, in gen- 

 eral, for all living organisms, including Man. Each starts its 

 independent existence as a single egg cell which, when fertilized 



