THE MITOTIC CYCLE 



differential centrifugation of a homogenate obtained by mechanical 

 disintegration (Claude^* 25 26^ . ^^g literature in this field has recently 

 been reviewed by Bradfield^' (1950) and by Schneider and Hoge- 

 BOOM.^^ It seems now quite certain that with due precautions, mito- 

 chondria can be isolated unchanged in form and in affinity for Janus 

 green (Hogeboom, Schneider and Palade^s), and with much of their 

 enzymic activity still intact. The mitochondria of the frog's eggs have 

 been shown by Recknagel^^ to contain cytochrome oxidase and 

 adenosine triphosphatase. Further researches in which these and other 

 enzymes of the cyclophorase system have been found to be associated 

 with mitochondrial fractions are described by Schneider^o and by Har- 

 MAN.^i In a succeeding chapter, Dr Waymouth (p. 1 79) refers to the use 

 of fractionation procedures for the isolation from the cytoplasm of parti- 

 culate material of lower orders of size. It is not yet certain how far the 

 'microsomes' prepared by these methods correspond with constituents 

 of living cytoplasm. In electron micrographs of cells, either of tissue 

 cultures or thin sections of liver, the cytoplasm has a granular texture, 

 in which particles of the order of ioom(jL in diameter can be discerned. 

 Claude has suggested that these correspond with the microsomes of 

 this range of size sedimented at the appropriate speed from cytoplasmic 

 fractions. 



Although no heterogeneity in 'clear cytoplasm' has yet been revealed 

 by optical methods, it is not impossible for particles of the size of 

 lOomfjL to be revealed by light microscopy. If there were sufficient 

 difference in refractive index in the living cell between microsomes and 

 their cytoplasmic background, they should be visible in the ultra- 

 violet phase microscope which has recently been developed (Taylor^^) . 

 Such observations would be of great interest and importance. 



FINE structure 



The nature of the sub-microscopical structure of protoplasm has been 

 discussed from several points of view. We will first consider the evidence 

 from what have been known as the so-called 'structure proteins' 

 which have been isolated from cells. These substances are fibrous 

 in nature, with properties analogous to those of myosin. In solution 

 they are highly viscous. The literature in this field has been reviewed 

 by Schmitt.^^ Bensley^* extracted 'plasmosin' from liver with 10 per 

 cent sodium chloride, while Banga and Szent-Gyorgi^^ prepared 

 'renosin' by extraction of kidney tissue with 30 per cent urea. It has 

 been subsequently shown that neither of these products is exclusively 

 cytoplasmic in origin, for deoxyribonucleoproteins are found in both, 

 as was shown by Lazarow^^ for plasmosin, and by Bracket and 

 Jeener'^^ for renosin. In these tissues, therefore, it has not yet been 



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