578 



GABRIELE KASS-SIMON 



upper stalk are not significantly différent from each other but each is significantly 

 greater than the number regenerated at the 25% level. The time at which basai 

 plates regenerate appears not to be significantly différent in the régions encom- 

 passing 75%, 50%, 25% and 10% levels. The différence occurs only when the 

 rate at each end of the hydra is considered with respect to the rates in the middle 



TRF 



300 _ 



200 



100 



75% 



50% 



25% 



10% 



AXIAL 

 LEVEL 



Fig. 4. 



Tentacle régénération factor at various axial levels. The mean TRF's of the weighted four 

 clone means at the given axial levels. (TRF=Tentacle Régénération Factor, JBT=Just 

 Below Tentacles. The Connecting line is intended only to show that the décline is not steady). 



body sections. And that (3) basai plate and hydranth régénération are not to be 

 considered mirror images of one another, reflecting opposite alternatives to a 

 single process manifesting itself in a single régénération gradient. (See Figure 6). 

 (This last statement will be further substantiated in Section VI of this paper). 

 And it is clear that whatever processes are involved and whatever mechanisms are 

 responsible for thèse axial différences, the différences are not sufficiently explained 

 by concepts either of a diffusing inhibiting substance or of adjacent dominance, 

 if each concept is taken strictly as postulated. 



2 In a recentiy published paper, Mookerjee and Bhattacharjee (1967, Roux' Arch .f. 

 Entwick.mech. 158: 301-314, " Régénération Time at the Différent Levels of Hydra "). report a 

 séries of experiments with Hydra vulgaris similar to those given above concerning the times of 

 basai plate and tentacle differentiation. In the présence of basai plate and hypostome, essentially 



