EXPERIMENTAL ANALYSIS 



reproduction of their own kind, but differ from viruses in their abihty 

 to undergo a directed differentiation into the subcellular functional 

 units of Stern. As Claude^ ^^ suggests, there are two alternative 

 hypotheses which could account for the reproduction of the great 

 variety of known cytoplasmic structures. Either each separate kind of 

 element is self-reproducing, or it is produced by an independent agency 

 v/hich must itself be self-perpetuating. As a working hypothesis Claude 

 chooses the second alternative, which permits the greatest economy of 

 assumption, namely that the 'existence of a reservoir of self-perpetuating 

 microsomes from which the specific granules may develop would pro- 

 vide a satisfactory answer to this problem'. Self-perpetuation is, in his 

 view (Claude^^*'), more likely to be the result of 'rigidly ordered chains 

 of reactions' and biochemical cycles than of the self-duplication of 

 highly complex substances by any kind of template replication. It is 

 not excluded that the microsomes may take their origin in the nucleus 

 (Caspersson and Schultz;1'i Schneider and Hogeboom;1^^ Weiss ;1'^2 

 Jeener and Szafarz^'^^) which Weiss believes may be the ultimate 

 source of all cytoplasmic systems. In leukaemic spleen, in which the 

 total pentose nucleic acid is 2*5 times the normal value, a parallel 

 increase is found in the smaller submicroscopic particles and in 

 the nuclei (Petermann et alii^'^^). Weiss^^* postulates that 'what 

 have appeared to be self-reproducing bodies in the cytoplasm would 

 simply become model centers of adsorption, aggregation, alignment 

 and conversion', originating in the nucleus but differentiating in the 

 cytoplasm. He agrees with Claude in proposing that such bodies are 

 'self-perpetuating' rather than 'self- reproducing'. Extracts of embryonic 

 tissue display much greater activity, perhaps in stimulating mitosis, cer- 

 tainly in promoting 'growth' and enhancing metabolic activity, than ex- 

 tracts of adult tissues (Carrel and Ebeling^"^ pp 1 73, 1 74) . It is possible 

 that the particulate fractions, and especially the smallest particles, which 

 arc much more abundant in embryonic cells, may be associated with the 

 activity. Bracket^ '''^ asks the question: 'A-t-on des raisons de penser 

 que les particules deplacables par ultracentrifugation jouent un role 

 dans la mitose ?' and gives evidence which leads him to an affirmative 

 answer, while Claude^'" ^'^'^ draws attention to the possibility that 

 anaerobic respiration, mediated by the microsomes and the nucleolus, 

 supplies at least part of the energy for the exacting requirements of 

 growth. 



Conclusion 



The older cytologists studied the static morphology of the cell at the 

 different stages of mitosis. Today cells are being studied dynamically 

 and in relation to their environment. For, 'the living cell is an unstable 

 system which can have neither existence nor form except in relation to 



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