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DISCUSSION 



STREHLER: How was it determined that cells move up into 

 the tentacles and die at the tips? 



BURNETT: Semal-Van Gansen (1951) vitally stained limited 

 areas of the tentacles and watched the stain migrate distally. 



STREHLER: Do both the gastrodermal and ectrodermal layers 

 move towards the tip of the tentacle? 



BURNETT: Yes, both move. In 1926 Issajew observed that if a 

 fork forms in the tentacle of a hydra, the fork will move distally 

 becoming progressively smaller until it gradually disappears at 

 the tentacle tip. We have observed this many times. Such a phenom- 

 enon would not occur unless both cell layers were migrating. More- 

 over, cnidoblasts are steadily pushed into the tentacles through such 

 growth processes from the sub-hypostomal growth region. 



STREHLER: Then the cnidoblasts don't migrate as free cells? 



BURNETT: In Pelmatohydra oligactis, there are always free 

 nematocysts in the enteron and in the digestive cells of the tentacles. 

 This can be demonstrated in the following experiments. The proxi- 

 mal portion of a methylene blue stained animal is grafted to the 

 distal portion of an unstained animal. Under conditions of normal 

 feeding, stained nematocysts are not found in the epidermal 

 batteries, but only in the gastrodermal cells of the tentacles. How- 

 ever, if the nematocyst supply in the tentacle batteries is depleted, 

 then stained nematocysts are transferred from the digestive cells of 

 the tentacle to the epidermal batteries. It is impossible for stained 

 nematocysts to reach the tentacle through growth processes because 

 the stained nematocysts are all proximal to the growth region, and 



