412 PRINCIPLES OF EMBRYOLOGY 



Much could probably be learnt, also, from specially designed experiments 

 on simple examples of them. 



Until such time as further theoretical or experimental studies have been 

 made of the properties of such systems, one is left with a certain freedom 

 of choice as to what hypothesis seems adequate to explain the facts of 

 determination and modulation. It may well be that there is no need to 

 assume anything more than competitive interaction between autocata- 

 lytic processes leading from genes to gene products, and from the latter 

 to the fmal cytoplasmic constituents. 



Some authors, however, feel that a further factor tending towards 

 persistence of character is required, and suggest that this can be found by 

 invoking the presence of plasmagenes. This possibility was discussed in 

 the last chapter, where the conclusion was reached that gene-initiated 

 plasmagenes would seem the most likely kind to play a large role in 

 development, but that although there is no doubt that the cytoplasm 

 contains complex bodies of a roughly gene-like order, one requires more 

 evidence of their independence of nuclear control before accepting them 

 as plasmagenes. This additional evidence can, as far as one can see at 

 present, be sought only in a further study of the properties of the micro- 

 somes to which Brachet has attached so much importance in the synthesis 

 of proteins. Are they indeed the main site of protein production, as he 

 suggests? And if so, have they in addition the property of multiplying 

 and retaining their own specific character in some degree of independence 

 of the genes? If so, one might allow that they were plasmagenes of one 

 or other of the types indicated in Fig. 18.2, Really conclusive evidence 

 on these points will, probably, only become available when the technical 

 difficulties of transplanting microsomes from one cell to the other are 

 overcome, and their degree of autonomy over against the nucleus can 

 thus be investigated. In the meantime, the best evidence we have as to the 

 developmental functions of microsomes comes from the phenomena of 

 evocation. 



In evocation we are undoubtedly confronted with a situation in which 

 an environmental influence, impinging in the first place on the cytoplasm 

 of the competent cell, causes it to adopt one or other of the alternative 

 paths of development open to it. The fact that the reacting tissue retains 

 its own specific characteristics — T. alpestris ectoderm forming typical 

 alpestris neural tissue even if evocated by T. cristatus mesoderm — shows 

 that the developmental paths are under genetic control and that the 

 evocation involves the differential activation of a particular set of genes. 

 The problem of the mechanism of evocation therefore becomes that of 

 the nature of the influences which can activate or inhibit genes; that is to 



