Fission and Gemmation in the Animal Kingdom, 37 



and is therefore local, is undeniable ; hut it is not every 

 instance of local growth that signifies gemmation. It is neces- 

 sary to exclude, firstly those regenerations which are localized 

 upon zones of growth, and secondly the large number of 

 processes of growth which, whetiier it be in consequence of 

 simple elongation, or whether it be due to actual increase in 

 bulk, are hereby restricted to an axis of the body (longitu- 

 dinal axis). This course involves nothing that is arbitrary, 

 but is rather a consequence of a logical necessity, since tliat 

 increase in size represents the normal form of growth of the 

 Metazoa in question and takes place even in those cases where 

 no asexual reproduction is combined with it. 



As regards the mutual relations of regenerations, separa- 

 tion, and growtli in the course of a case of paratomy, I have 

 already mentioned the variation which occurs in the time of 

 the ajipearance of separation. With reference to this we 

 might distinguish cases of paratomy with precocious regene- 

 rations from those in which they are of subsequent occur- 

 rence, were it not for the existence of the difficulty which is 

 due to the fact that in many cases separation sets in when 

 the first stages of the new formations have already com- 

 menced. 



The relation in time between the regenerations and growth 

 is here of special interest for us. In this respect the fission 

 of the Naids is perhaps the most instructive and may serve 

 as an example. 



In the first place the growth of the Naid in process of 

 fission appears everywhere as segmental and restricted to the 

 longitudinal axis of the body of the animal, as is typical for 

 the segmented worms ; it is therefore an individual growth. 

 But the extent of the increase in size, which is for the time 

 being attained by the fission- products which are in process of 

 formation, varies greatly, owing to the fact that the regene- 

 rations, that is the zones of regeneration, already appear 

 before the growth of the zooids which are originated thereby 

 has developed a trunk-section of any size (reproduction from 

 the anal somite) ; or, in other words, that the point of time at 

 which the rudiments of the zones of regeneration are deve- 

 loped appears to be transferred to constantly earlier stages in 

 the size and therefore in the development of the future 

 zooids. In consequence of such accelerations it is easy to 

 form the impression that the fission-product grows out as a 

 bud from the parent form. In connexion with forms of para- 

 tomy in the Naids which run a more regular course, however, 

 these alterations in the order of time will become of so much 



