POLYPI. 125 



cesses of development in the Hydra, appear to be the chief conditions 

 influencing that modification of the generative process by whicli a 

 small portion only of the Hydra is taken into the system of the new 

 individual, instead 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 ova, the presence of the pellucid nucleus of a 

 secondary impregnated 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. It even 

 presents a periodical development of sexual organs of two kinds; one, 

 at the anterior or oral extremity of the body, consists of small nodules 

 or sacs, which Ehrenberg discovered to contain moving filaments, or 

 seminal animalcules ; another series of cells, developed in the pos- 

 terior part of the stem, contain ova. Sometimes one individual 

 Hydra developes only the male cysts, or sperm-vesicles ; sometimes 

 only the female ones, or ovisacs ; but the rule is generally to have 

 both kinds. 



The ova are spherical, with a bristled chorion, which is of a deep 

 brown colour in the Hydra fusca. In the formation of the ova, certain 

 of the retained germ-cells multiply themselves, and coalesce to form a 

 larger central cell, surrounded by others of smaller size, with nuclei, 

 the exterior of which cells 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, and they 

 develope a hydra, retaining, however, a large proportion of un- 

 changed cells in its composition. Accordingly, this hydi-a may 

 propagate by buds, and the hydra so developed may propagate again 

 by ova, and these two kinds of generation may alternate indefinitely : 

 but it usually happens that the same Hydra, after having exhausted 

 its power of forming buds, then developes the eggs. 



The seas which wash our own shores are tenanted by numerous 

 forms of minute Polypi, having essentially the same simple organisa- 

 tion as the Hydra ; but which are protected from the dense briny 

 element by an external horny integument. Now these likewise 

 develope new polypes by gemmation ; but, as the external crust 

 grows witli the growth of the soft digestive sac, the young polype 

 adheres to the body of the parent, and, by successive gemmations, a 

 compound animal is produced. Yet the pattern according to whicli 

 the new polypes and branches of polypes are developed is fixed and 

 determinate in each species ; and there consequently results a par- 

 ticular form of the whole compound animal or individual by wliich 

 the species can be readily recognised {fig. 61.). This compound 



