148 THE BIOLOGY OF HYDRA : 1961 



DISCUSSION 



WOOD: Have you seen a periodicity in the internal supporting 

 structures which extend down past the capsule itself? I have seen 

 this several times, a periodicity reminiscent of the periodicity of 

 the ciliary rootlet found in other organisms but much narrower. I 

 was curious whether you had made a similar observation. 



CHAPMAN: I have not seen any periodic structure there. 



FAWCETT: For the benefit of those who are not electron 

 microscopists I would like to state that these pictures represent a 

 notable technical achievement. I think they show very clearly what 

 dramatic progress has been made in this field in a few years, largely 

 as a result of the introduction of new imbedding materials. A few 

 years ago, with all Dr. Chapman's skill, it was just impossible to 

 get such fine pictures of this very difficult object. Now with epoxy 

 resins, one can get beautiful micrographs of nematocysts and other 

 cytological features of hydra. 



With respect to the localization of collagen or collagen-like 

 material— don't you feel that the very fine filaments found through- 

 out the substance of the nematocyst capsule may be tropocollagen 

 or collagen in such a state of dispersion that it would not be 

 expected to exhibit any periodic structure? The area which you 

 showed to have periodic cross banding may simply be a result of 

 side-to-side aggregation of such macromolecular units of collagen 

 that are more widespread than this limited occurrence of cross 

 striated structure would suggest. 



CHAPMAN: This is very possible. 



LENHOFF: The "collagen " of H. littoralis nematocysts is different 

 in many respects from vertebrate collagen. For example, there is 

 much more hydroxyproline and proline and also less glycine in the 

 nematocyst capsule than in classical collagens so that it is not sur- 

 prising that the periodicity is different. In fact, we are very 

 gratified that you find any periodicity at all. 



WELSH: I wonder if this interesting structure that you show near 

 the tip of the external tubule might conceivably be a supporting 

 structure? I would seriously doubt that it was the toxin. Do you 



