MULTIPLICATION BY GEMMATION 157 



I therefore speak of it as ' blastogenic idioplasm.' It cannot be 

 stated with certainty which cells of the ectoderm contain this 

 idioplasm ; it seems, however, that the growth of the bud 

 originates in the deeper layer, i.e., in the 'interstitial' cells. 

 We may therefore suppose that some of these interstitial cells 

 contain inactive blastogenic idioplasm, which, after a certain 

 series of cell-divisions necessary for the growth of the polype, 

 obtains the control of one of the offspring of these cells, and so 

 causes budding to take place. Each bud must originally arise 

 from one cell only, although this fact has not as yet been actually 

 proved ; and in the first division, or at any rate in the early divis- 

 ions of this cell, the group of determinants of the ectoderm must 

 become separated from that of the endoderm, the 'bearers' of 

 the latter group migrating into the old endoderm through the 

 disintegrating supporting lamella. The remaining details of the 

 process require no further explanation. 



In the Hydrof/iediisce, then, each bud originates in a single 

 cell, and thepjrocess of multiplication by gemmation therefore 

 differs essentially from that of reproduction by fission. For 

 gemmation owes its origin to the entire mass of the determinants 

 of the species, which only undergo disintegration at a later stage ; 

 while the new structures which arise by fission originate simul- 

 taneously from numerous smaller groups of determinants, corre- 

 sponding with those of the later stages of ontogeny. 



We should nevertheless be mistaken in supposing that the 

 essential difference between fission and gemmation is due 

 altogether to this difference as regards the group of determinants 

 concerned in the two processes. This is rendered evident by a 

 comparison with the processes of budding in other groups of 

 animals. It still remains to be shewn whether the process of 

 gemmation in other Coelenterates, viz., in the Actinozoa, the 

 higher Mediisce, and the Ctenophora, also only apparently origi- 

 nates from both layers of the body-wall, or whether it actually 

 arises from one layer only. As the possibility of the latter mode 

 of origin has not till now been considered, it is very possible that 

 the migration of cells may have been overlooked in this case also. 



If we now turn our attention to the other groups of the animal 

 kingdom in which gemmation occurs, viz., to the Polysoa and 

 Tunicata, we shall find that we possess the results of very excel- 

 lent investigations on which our arguments can be based ; and 

 the histological structure of these animals is such as to render 



