IV REGULATING FACTORS OF THE PFS 519 



development. The second set of processes may concern the selective control of the 

 bulk production of a given set of cell proteins (and consequently of other cell prod- 

 ucts) characteristic of the cell types. 



Before accepting this dichotomy as a basis for a rigid classification of develop- 

 mental processes and for a hypothetical distinction between mechanisms involved, 

 the limitations of such a classification have to be clearly recognized. 



It is well known that at very early stages of development the PFS even of 

 "mosaic" eggs will exhibit some degree of lability. On the other hand, in regulative 

 development some properties of the cells of embryos may become stabilized while 

 others remain labile for a longer time. This eliminates the distinction between 

 these two types of development in principle. 



Due to the limitations of our analytical methods, it has not been established 

 beyond any doubt that apparent qualitative diflferences found between embryonic 

 cells are not merely the expression of very large quantitative differences. For 

 example, during early amphibian development the undifferentiated mesodermal 

 cell can respond to environmental factors by forming either hemoglobin or muscle 

 protein (Yamada, 1940 and Muchmore, 1951). At a critical period in the devel- 

 opment of the primitive mesoderm the cells initiate bulk production of only one 

 of these proteins and become either blood or muscle cells. Once this change has 

 occurred, these cells do not any longer respond with production of the alternate 

 protein type to the same environmental conditions which elicited formation of 

 such proteins during an earlier stage of development. However, it cannot be said, 

 with absolute certainty as yet, that reversibility of this change is excluded under 

 all experimental conditions or that the fraction of the PFS which can be induced 

 during early phases of development to form an alternate protein species {e.g. 

 hemoglobin) has become completely and irreversibly inactivated and does not 

 merely function at a rate too low to allow detection of its product without special 

 methodology. 



Only with these reservations in mind have we discussed separately those factors 

 which seem to control the development of the qualitative aspects of the PFS and 

 the factors which control quantitatively the production of protein. It may remain 

 impossible, for reasons of the limitations of analytical procedures, to distinguish 

 between these aspects. 



(a) Apparent qualitative changes in the PFS [Induction) 



Long before the nature of the PFS was, or could be, considered as a key to the 

 mechanism of differentiation, the factors influencing qualitatively the formation 

 or function of the PFS had become the main issue of experimental embryology. 

 With Spemann's analysis of the "Organizer" it became apparent that some forms 

 of interactions between mesodermal cells and the ectoderm lead to the trans- 

 formation of the latter into the parts of the brain and neural tube (summarized 

 by Spemann, 1938). This discovery started an intense search for the biochemical 

 mechanism of the phenomenon of induction. Initially it was assumed that a 

 single specific substance was produced by the mesodermal cells which was 

 responsible for the transformation of an undifferentiated ectodermal layer into 

 the highly specific neural tissue. Reports that inductions had been obtained with 



Literature p. 53g 



