10 



on the processes of development in the Hydra, appear to 

 be the chief conditions influencing that modification of the 

 generative process by which a small portion only of the 

 Hydra is taken into the system of the new individual, in- 

 stead of one-half of the body, as in the case of the Monad. 

 So insignificant is the distinction between gemmation and 

 spontaneous fission ; the essential condition of both being, 

 as in the development of the ovum, the presence of the 

 pellucid nucleus of a derivative germ-cell, as the centre 

 from which all the processes in the formation of the new 

 individual radiate. 



The Hydra propagates by ova as well as by buds. Cer- 

 tain of the retained fertile germ-cells multiply themselves, 

 and coalesce to form a larger central cell, surrounded by 

 others of the ordinary smaller size, the exterior of which 

 are metamorphosed into a chorion. Certain other germ- 

 cells are converted into sperm-cells, and develope sperma- 

 tozoa. The ova are extruded and fertilized by these: each 

 constitutes the first term of a new series of life and deve- 

 lopes a Hydra, which retains however a large proportion 

 of unchanged cells in its composition. Accordingly this 

 Hydra may propagate by buds, and the Hydra so de- 

 veloped may propagate again by ova, and these two kinds 

 of generation may alternate indefinitely. 



In most of the marine hydriform polypes, the delicate 

 tissue of the body is supported in the waves and breakers, 

 and protected from the briny element by an external horny 

 integument. Were such a polype to propagate by gem- 

 mation, and the external crust to grow with the growth 

 of the bud, and expand to protect the soft digestive sac 

 of the new polype, an apparently compound animal would 

 result from the number of individuals so held together. 

 Such is the nature of a vast family of marine zoophytes 

 which our Lyncean Ellis has so accurately described and 

 beautifully figured as ' Corallines */ and which are grouped 



* An Essay towards a Natural History of Corallines, &c., 4to. 1755. 



