THE MITOTIC CYCLE 



the smallest particles containing the highest proportion of pentose 

 nucleic acid and the lowest enzyme content. The smallest particles, in 

 common with virus particles, seem to possess no enzymatic activities. 

 Embryonic tissues contain a larger proportion of pentose nucleic 

 acid not sedimented at high speeds than do adult tissues (Jeener and 

 Brachet^^^). Suspensions of granules from 2-day chick embryos, or 

 from frog neurulae, applied to the chorio-allantoic membrane of the 

 chick embryo, produced marked thickening (Shaver and Brachet^^^). 

 The results of irradiating with ultraviolet, and of heating the material, 

 were not conclusive. The integrity of the granules is hardly likely to 

 survive heating for lo minutes at 80° C, treatment which did not always 

 cause inactivation. It was therefore thought unlikely that the particles 

 multiply in the membrane as viruses do, and Shaver and Bracket 

 suggest that the particulate material contains some constituent which 

 stimulates pentose nucleic acid synthesis. The concept of normal cyto- 

 plasmic particles penetrating normal cells and behaving like a virus is 

 nevertheless developed by Brachet,i^* who had already^ ^^ stressed the 

 probable importance of nucleoprotein-containing particles as agents of 

 protein synthesis. 



Enzymatic activity 



MooG and Steinbach^^^ ^^' studied the enzymatic activities of gran- 

 ules isolated from chick embryo cells, on the hypothesis that 'incorpora- 

 tion of enzyme into these granules during the embryonic period would 

 ... be an aspect of the building up of an 'activity structure', a funda- 

 mental sort of differentiation on which more overt differentiation may 

 be founded'. The evidence shows that embryonic 'development involves 

 the construction of increasingly complex associations of biochemical 

 constituents, and it seems reasonable to assert that, from an embryo- 

 logical point of view, the most important enzymes are those which 

 become linked with, and presumably oriented upon, such granular 

 associations.' Cellular differentiation is conceived (Brachet^^^ ^^^) as 

 a process associated with the development of increased size, organiza- 

 tion and complexity of the cytoplasmic granules, and a differentiated 

 cell as one which has acquired a certain stability. There may be 

 stability at two levels: 'dynamic' stability of particle population, 

 resulting in stable morphological and functional characteristics in the 

 cell ; and stability of differentiated particles in each cell, wdth conse- 

 quent loss of pluripotentiality and of ability to initiate reduplication. 

 It is suggested that the metabolic and synthetic activities of the cell 

 may be stimulated or controlled by the pluripotent undifferentiated 

 small particles in a direction which leads to cell division. The hypothesis 

 rests on the unproven assumption that the smallest particles, character- 

 istic of embryonic tissues, resemble viruses in their ability to promote 



180 



