388 PRINCIPLES OF EMBRYOLOGY 



transmissable viruses. When from such evidence we deduce the existence 

 of a plasmagene, it is presumably implied that the cytoplasmic determin- 

 ant 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 

 has an appropriate competence, a particular type of cellular differentia- 

 tion 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 

 reahsed 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 

 simulate a mutation of the virus. If the extracts came to contain free 

 ammonia they would transform the gastrula ectoderm not into neural 

 tissue but into derivatives of the axial mesoderm. 



More recently Lederberg (1952) has drawn attention to the same 

 source of possible error. Again, Pollock (1953) has suggested a mechanism 

 for the operation of self-perpetuating and even growing systems which 

 might very easily be confused with plasmagcnes (Fig. 1 8.1). In the form he 

 advances it, the idea depends on the phenomenon of enzymatic adaptation 

 (p. 400) ; but somewhat similar systems might be produced in other ways. 

 Pollock's suggestion is this; suppose that a substance A, supphed to a cell, 

 causes it to synthesise an enzyme a which converts ^ to B; then suppose 

 that B induces the formation of enzyme b, which converts B (perhaps in 

 combination with other substances already in the cell) into C; then that 

 similarly C is converted into something else, and that fmaUy a product 

 is produced wliich is converted again into A. Such a system will be set 

 going by the addition of an initial quantity of ^ and will then carry on 

 indefinitely. In fact if the system absorbs energy, more A may be formed 

 at the end of a cycle than entered it at the beginning, and the system 

 will be able to grow. Such a system clearly has many of the properties 

 attributed to plasmagenes; but it need not be incorporated in a particle. 

 Thus to be justified in using grafting or inoculation experiments to 

 postulate the existence of a plasmagene, one needs evidence not only that 

 the character can be transmitted by cell-free extracts but that the effective 

 factor in the extracts is a particle of the right order of complexity. 



Discussions which on the whole favour the importance of plasmagenes 

 in development v^ be found in Darlington 1944, Darlington and Mather 



