168 A. S. Parkes 



sity for a comparatively high concentration of glycerol or 

 other neutral solute for the freezing of individual cells and 

 tissues. An approach to the problem has, however, been 

 made possible by the work of And jus (1951) who evolved a 

 method by which rats could be cooled to deep body tempera- 

 tures approaching 0° C. and revived again after cessation 

 of heart beat and respiration for one or two hours (Andjus 

 and Smith, 1954). Application of this technique to hamsters, 

 chosen because of their known adaptability to body tem- 

 peratures in the range between 2-5° C. and 38 °C., enabled 

 work to be concentrated on the effects of reducing body 

 temperature below zero (Smith, Lovelock and Parkes, 1954). 

 Intriguing results were obtained. As the deep body tempera- 

 ture goes below zero, one or other of two things happens, 

 (a) Some hamsters start to freeze peripherally and the deep 

 body temperature stabilises at about —0-6° C. when the 

 surrounding bath is held at — 5°C. to — 7°C. Crystallisation 

 of body water proceeds slowly but steadily and the extremi- 

 ties and superficial tissues become hard frozen. Complete 

 recoveries have so far been effected after periods of freezing 

 up to one hour, by which time it can be calculated from the 

 rate of heat loss that not less than 15 per cent of the body 

 water has crystallized, (b) Other hamsters start to super-cool 

 as the deep body temperature goes below zero, and the tem- 

 perature falls steadily until it approaches that of the bath. 

 Deep body temperatures as low as — 5°C. without crystalliza- 

 tion have been recorded, and such animals can be resuscitated 

 completely. More often crystallization takes place suddenly 

 from the super-cooled state, accompanied by an abrupt rise of 

 temperature to about — 0-6°C. CrystalUsation arising in 

 this manner appears to be diffuse throughout the body and 

 is more damaging than that arising peripherally from gradual 

 freezing. 



These experiments, though highly intriguing, touch merely 

 the fringe of this subject, and it must be emphasized that 

 there is no immediate prospect of retaining viability in whole 

 animals frozen to very low temperatures. 



