440 THE BIOLOGY OF HYDRA : 1961 



as an individual interstitial cell or after the cluster has completed its 

 differentiation and separates again into individual cells. It would be 

 quite impossible for a syncytial cluster of 8 or 16 cells connected 

 by these bridges, which I believe in very firmly, to insinuate them- 

 selves between other cells in the column or to cross the mesoglea 

 and get into the gastrovascular cavity and re-invade at a higher 

 level. I think this syncytial relationship almost excludes any migra- 

 tion in the interim period. They will either have to migrate as un- 

 differentiated individual interstitial cells, or as cnidoblasts that have 

 matured nematocysts within them. 



STREHLER: I wonder whether you've ever seen migration of 

 any nematocysts or cnidoblasts into the tentacles of Campanularia. 

 In this case they could not move through the gastrovascular cavity 

 because the tentacles have no cavity. Also, I would like to ask why 

 you call that pigment lipofuscin. 



BURNETT: The inclusions found in Hydra pirardi fed on Ar- 

 temia are not carotenoids and are not dissolved by lipid solvents. 

 They are dense bodies, often found in clusters, and as I remember, 

 they stain only after they have been oxidized by permangenate or 

 a similar oxidizing agent, 



STREHLER: If it was not extractable with organic solvents, then 

 perhaps you should call it "hydrafuscin." What were those slides 

 stained with? 



BURNETT: Methylene blue. 



STREHLER: Was that the natural color of those granules? 



BURNETT: Yes, the methylene blue didn't go into the granules. 



STREHLER: Do you find them down toward the base? 



BURNETT: Yes, like the carotenoids in hydra, they are especial- 

 ly concentrated in the hypostome, budding region and basal disk. 

 They probably represent some type of excretory crystal. They per- 

 sist for a greater length of time during starvation. I am not quali- 

 fied to comment any further on their nature or function. 



