384 ZOOLOGY. 



bud in that the mesoglcea and endoderm take no part in it. 

 If the gonad is to become an ovary, one of the interstitial 

 cells becomes amoeboid and increases in size by feeding on 

 the others. Yolk-spheres are stored up in it, and also 

 (in H. viridis) chromatophores a remarkable fact in a cell 

 of ectodermic origin. Eventually the ovum, for such it is, 

 ceases growth and becomes spherical, its nucleus divides 

 twice, and the usual polar bodies are cut off. About this 

 time the layer of ectoderm cells that had been covering 

 it hitherto splits, and the ovum is exposed to the water. 

 It is now a large spherical body, about a millimetre in 

 diameter, often more conspicuous than the Hydra that 

 bears it. 



Meanwhile the interstitial cells in the testes have been 

 dividing to form spermatozoa of the ordinary form. These 

 escape and swim about, one of them eventually meeting the 

 ovum and fusion of the pronuclei taking place. 



An animal in which, as in Hydra, both ova and 

 spermatozoa are produced on the same individual is said to 

 be hermaphrodite. In many hermaphrodite animals there 

 is provision of some kind for making cross-fertilization (i.e. 

 the fusion of pronuclei derived from separate individuals) 

 easier than self-fertilization, but no such provision is found 

 in Hydra. The fertilized ovum segments until a hollow 

 blastosphere is formed. The single cell-layer then becomes 

 divided into two, and a two-layered stage is produced, 

 differing from a gastrula in the absence of a blastopore. 

 From this point development is simply a matter of differen- 

 tiation. 



12. Somato-plasm and Germ-plasm. Thus, while in 

 asexual reproduction ectoderm gives rise only to ectoderm, 

 and endoderm only to endoderm, in sexual reproduction a 

 cell derived entirely from ectoderm gives rise to a complete 

 Hydra, consisting of both ectoderm and endoderm. Evidently 

 the nuclear material of the interstitial cells of the ectoderm 

 contains, in addition to what we may call its " ectodermal 

 potentiality," a further " full Hydra potentiality " which 

 becomes active as a result of the sexual process. According 

 to the theory of Weissmann, these two potentialities belong 



