THE DISCONTINUOUSLY ZOOID AL 73 



by the ectoderm at a point near the distal end of the zooid 

 in a little excrescence called the " testis," while the latter 

 usually appear as a single ovum in a little sac nearer the 

 proximal end of the zooid. Both spermatozoa and ovum 

 are set free in the water and fertilisation occurs in this 

 medium. The fertilised ovum lies in the mud till the return 

 of warmer weather, when it proceeds to become the first 

 zooid of a new Individual. 



The formation of a new zooid by gemmation is an act of 

 production, not of reproduction, for the zooid is a part of, 

 or a stage in the development of the Individual ; and while 

 in a budded zooid there is a reproduction of structural plan, 

 our belief is that each zooid represents a cyclic advance on 

 its producer. We might say that the budded zooid carries 

 on the cycle from where its producer temporarily halted, 

 or that as each vegetative cell of the discontinuously multi- 

 cellular Individual is a stage on the road towards the end 

 of the cycle, so in a sense is each budded zooid, in a straight 

 line of " descent." The compensation for the discontinuity 

 of any growth-unit is the power of carrying the cycle to the 

 end, or of becoming sexual, and thus each Zooid of the 

 Hydra Individual should under favourable conditions attain 

 to sex. But, on the other hand, the price of Continuity is 

 Arrest, and thus it is that the great majority of the cells 

 composing the zooid never become sexual, in their " descend- 

 ants " ; they live not for themselves, but for the good of the 

 zooid as a whole, and their arrest enables the zooid to persist 

 and retain its identity. 



In Hydra only a certain number of cell-lines are allowed 

 to terminate, and to do this along an interrupted path. And 

 it is not unreasonable to suppose that it is through the cell- 

 division involved in repeated gemmation that certain areas 

 of a zooid finally produce sexual elements. For, in the 

 first place, it is only through cell-growth, division, and 

 multiplication, that sexual elements can ever appear in any 

 cycle whatsoever. Again, nowhere, after zooidal maturity 

 is reached, does repeated cell-division occur except at 

 gemmation areas ; elsewhere there is cell-arrest. The natural 

 conclusion one tends to arrive at is : — 



I. That a given zooid has to bud others in order to become 



