422 THE BIOLOGY OF HYDRA : 1961 



These postulated mechanisms are very speculative, but they do 

 offer a basis for planning future investigations concerned with 

 the mechanisms of interstitial cell differentiation and of mesogloeal 

 restoration during regeneration and the interplay of these two 

 factors as a growth controlling mechanism in regeneration. 



CONCLUSIONS 



Many factors (both intrinsic and extrinsic) influence the differ- 

 entiation of interstitial cells to cnidoblasts during regeneration. 



Other cell types must be considered as possible contributors 

 to the process of interstitial cell differentiation. 



The mesogloea is not visible in areas where intense interstitial 

 cell differentiation is proceeding in normal hydra or severed hydra 

 ( both in untreated and lipoic acid treated organisms ) . 



A growth regulating mechanism is proposed wherein a balance 

 between the quantity of mesogloea present in an area of the 

 hydra and the extent of cell differentiation (apparently brought 

 about by the close contact of the gastrodermis and the epidermis) 

 must be achieved for normal regeneration to occur. 



Although the emphasis has been placed upon the importance 

 of cell migration, cell differentiation and the interaction of cell 

 layers during the regenerative process, other mechanisms (many of 

 which are still unknown) must also contribute to this very complex 

 process. 



A knowledge of the interactions occurring throughout the whole 

 animal, especially the interrelationship l^etween difterent cells and 

 cell layers through chemical and physical interchange, must be 

 acquired before a true understanding of the overall process of 

 regeneration will be achieved. 



REFERENCES 



1. Brien, p. and M. Reniers-Decoen, 1949, La croissance, la blastogencse, et 



I'ovogenese chez YHijdra fusca (Pallas). Bull. biol. France et Belg. 83: 

 293-386. 



2. Burnett, A. L. 1959. Histophysiology of growth in hydra. /. £.v/). Zool. 140: 



281-342. 



