General Discussion 171 



that ultimately conditions permitting the indefinite survival of a great 

 variety of tissues will be obtained. In tlie original experiments with red 

 blood cells, freezing and thawing as such caused the loss of about 5 per 

 cent of the cells but about 40 per cent were lost during storage for six 

 months. But improvement of conditions, particularly the use of 

 hypotonic media to reduce the residual salt effect, has now enabled the 

 loss to be reduced to 1 per cent over a year or more. 



Cowdry: You said at the beginning that there seemed to be a difference 

 between persistence or the holding up of changes that were physical as 

 compared with chemical ones, that the physical changes might continue, 

 whereas the biochemical ones would be arrested. I would like to get a 

 little more information. For instance, does the half life of a radioactive 

 isotope change when the tissue is in a vitrified condition? 



Parkes: We don't have any information about that. 



Verzcir: I think you can get information on that point from tlie food 

 industry. If one stores fish, say herring, at — 20° they are edible; if one 

 stores them at —80° or so, they become inedible. The conservation of 

 proteins seems to depend on tlie temperature at which one stores. 



Parkes: Does that apply to herring soused in glycerol? 



Verzdr: I don't know. 



Dcnn: I'd like to report very briefly on some experiments Dr. Bunge 

 has done in Iowa; I can't give you the details but only the main facts. 

 He published in Nature some months ago a brief letter about the freezing 

 of human sperm, and then after a period they were thawed and given to 

 tliree women by artificial insemination. During the past six months each 

 of those women has given birth to an apparently perfectly normal child. 



Parkes: Well, of course, there's no reason to suppose the offspring 

 wouldn't be normal. Tens of thousands of calves have now been pro- 

 duced from frozen sperm, without, so far as I know, any indication of 

 sinister manifestations. 



Medawar: Dr. Parkes, you believe that delayed autografting is prefer- 

 able to the use of homoplastic transplantation methods as outlined by 

 Krohn, don't you? But at the same time you believe that transplanta- 

 tion immunity is greatly over-rated? 



Parkes: I don't think so, but my rats appear to! Even with inter- 

 strain ovarian homografts we get about 20-25 per cent which apparently 

 function indefinitely. Many of them function for a time. 



Medawar: Well then, for creating these age chimeras, would it not be 

 quicker and more satisfactory to devise a method wliich would make it 

 possible for homografts to be transplanted freely between different 

 animals of different ages? 



Parkes: Yes, that is another approach. I should emphasize that this 

 work of ours was not primarily designed to assist gerontology. 



Comfort: Prof. Medawar, I believe your results are better in fact with 

 grafts between mice of the same strain, in view of what you said, tlian 

 with stored rabbit material? 



Medawar: Yes. 



Comfort: If one had to work w itli the rabbit or dog, I wonder whether 

 artificial parthenogenesis would give you a source of material. 



