47 



end as in the Conferva. The mutual contact of these delicate 

 cells suffices for mutual action and reaction on one another ; 

 excess of nutriment or other quality in one is balanced by 

 the endosmotic action of neighbouring cells, just as the 

 nutriment is transmitted from cell to cell to the last deve- 

 loping-cell in the chain forming the conferval filament. 



The young Hydra from the bud is identical in organic 

 structure and character with that which comes from the 

 ovum : and when the effects of organic development are 

 the same, their efficient causes cannot be e altogether di- 

 stinct ' : only the non-essential accessories of the process 

 may be the subject of variation. 



The retention of unused-up germ-cells in the tissues 

 of the Hydra is the condition which explains the fact, 

 that, when favourable external circumstances of light, heat, 

 and food concur, it may be propagated by buds. It is not, 

 however, absolutely correct to say that buds " may arise 

 from any portion of the body of the parent structure;" in 

 the Hydra fusca, at least, they are pretty constantly de- 

 veloped from a special part of the animal, as was long ago 

 shown by Roesel*, and has been recognized by all sub- 

 sequent observers : the part in question is at or near to 

 the bottom of the digestive sac ; between it and what might 

 be termed the foot or attaching pedicle of the polype ; and 

 this is precisely the part where those nucleated cells are 

 assembled in greatest number, which most closely resemble 

 the secondary or derivative germ-cells in the ovum. 



It might be specified as the ( ovarium' of the polype, but 

 that the true ova are matured from cells not situated exclu- 

 sively at this part ; nevertheless the germ-cells situated be- 

 tween the stomach and pedicle are the essential elements of 

 ova, and are only not converted into ova, and cast off with a 

 chorion, because their development proceeds directly and 

 at once to the business of the formation of an embryo : and 



* See his beautiful 86th plate in the ' Historise Polyporum/ of the 

 Supplements to the ' Insekten-Beliistigiingen.' 



