124 LECTURE VII. 



The Hydras are not less remarkable for their power of generation 

 than for that of regenerating mutilated parts. They have been 

 observed to multiply by spontaneous fission, dividing themselves 

 transversely. Roesel* figures a specimen in the act of transverse 

 fission ; but there may have been some previous injury at the part. 

 The most ordinary process of generation is by the development of 

 young polypi, like buds, from the external surface of the old one. 

 This property depends on the small amount of change which the 

 germ-mass has undergone in the development of the body. 



In the freshwater polype, the progeny of the primary impregnated 

 germ-cell retained unaltered in that body, may set up, under favour- 

 able stimuli of light, heat, and nutriment, the same actions as those 

 to which they owed their own origin ; certain of the nucleated cells 

 do set up such actions, those, e. g. in the Hydra fusca, which are 

 aggregated near the adhering pedicle or foot ; and the result of their 

 increase by assimilation and multiplication is, to push out the con- 

 tiguous integument in the form of a bud, which becomes the seat of 

 the subsequent processes of growth and development ; a clear cavity or 

 centre of assimilation is first formed, which soon opens into the 

 stomach of the parent ; but the communication is afterwards closed, 

 and the young hydra is ultimately cast off" from the surface of the 

 parent. This mode of propagation is termed "gemmation." It diflfers 

 from the development of the hydra ab ovo, inasmuch as the im- 

 pregnated germ-cell, which set on foot the process, is derivative and 

 included in the body of the adult, instead of being primary and 

 included in a free ovum. But the germ-cell is the essential part of 

 the ovum, and the chorion an accessory and non-essential part.^ 



According to my observations, buds are not developed indifferently 

 from any part of the polype. I have never seen one growing from a 

 tentacle, nor does a wound of this part lead to the development of a 

 young hydra, like a wound of the base of the body. I conceive that 

 the greater amount of metamorphosis which the germ-cells and nuclei 

 have undergone in the formation of the complex organs of the 

 tentacula is the condition of this inferior power of generation and 

 regeneration. 



The very small size in relation to the entire body, and the super- 

 ficial position of the secondary germ- cell which takes on the pro- 



(^Hydra grisea), " qui sont restes retournes et qui ont longtemps vecu. Us ont 

 mange, cru ct multiplie." CI. t. ii. p. 224. 



* CV. tab. Ixxxiii. fig. 3. 



f Mr. Goodsir gives the name of ovule to the germ-cell which sets on foot the 

 analogous process of bud-formation in the Ccenurus (LIII. p. 570). 



