The cell physiology of early development 



particular histological character, for instance the myosin in a muscle cell, the secretory 

 enzymes in a gland cell, the proteins which give rise to keratin in an epidermal or 

 hair-forming cell, etc. Intermediate, as it were, between the raw materials and the 

 final cellular products there must be a number of other categories. About these we 

 know much less. There must, however, be substances which mediate the effects which 

 the genes produce. We may tentatively consider a category of 'immediate gene 

 products' given off by the genes and passing into the cytoplasm either at cell division 

 when the nuclear membrane is destroyed, or perhaps continuously. These must inter- 

 act with other substances already present in the cytoplasm. Possibly the immediate 

 gene products control the whole progression from the cytoplasmic raw materials to 

 the cellular final products, but it may be that the cytoplasm also contains other sub- 

 stances on the path between these two extremes not directly controlled by the im- 

 mediate gene products. 



Among the cytoplasmic constituents intermediate between the raw materials and 

 the final products there may, indeed, be entities with properties of a somewhat gene- 

 like nature. In recent years a great deal of study has been devoted to factors which 

 may in general be termed 'plasmagenes', and extremely important advances in our 

 knowledge of them have been made. The term 'plasmagene' covers a wide range of 

 different types of entity. It has often been suggested, though in somewhat vague 

 terms, that they may play an important part in cellular differentiation. It is worth 

 discussing at some length the different sorts of plasmagenes whose existence has been 

 demonstrated, and attempting to evaluate their possible role in embryonic develop- 

 ment. 



Broadly speaking, plasmagenes are revealed by two different types of evidence. 

 On the one hand, breeding experiments of the usual genetical kind may demonstrate 

 that certain characters are inherited through the cytoplasm and not through the 

 nucleus and thus provide evidence for cytoplasmic hereditary determinants. A 

 different type of evidence appears when it can be shown that a character can be 

 transmitted from cell to cell by inoculation or other treatment with extracts which 

 do not contain functional chromosomes; we may then conclude that we are con- 

 fronted with a determinant, presumably derived from the cytoplasm, which can 

 persist and impress some definite character onto the living cells into which it is intro- 

 duced. The classical examples of such types of behaviour are the transmissible 

 viruses. When from such evidence we deduce the existence of a plasmagene, it is 

 presumably implied that the cytoplasmic determinant is a fairly complex body, 

 probably of the order of magnitude of a virus particle or a gene. Considerable 

 caution should be exercised in making such deductions. Many years ago, in the early 

 years of the investigation of cancer-producing viruses, it was pointed out that, given 

 a tissue which had an appropriate competence, a particular type of cellular differen- 

 tiation could be transmitted through an indefinite series of inoculations by means of 

 cell-free extracts whose operative factors, however, were quite simple molecules 

 which acted as evocators (cf. Needham, 1936). One knows now that the effective 

 molecules might be even simpler than was realized at that time. It would be quite 

 possible to carry on an indefinite series of transformations of gastrula ectoderm into 

 neural tissue by means of inoculations of cell-free extracts, provided only that these 

 extracts were sufficiently acid. Moreover, one might easily obtain phenomena which 



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