288 Action of the Genetic Material 



susceptible to environmental interference. Thus we feel entitled to 

 explain development exclusively in terms of genie action, though using 

 modifying actions of the environment as tools which can help us to 

 understand normal genie action. We discussed this when analyzing 

 phenocopies, and when first using the concept of norm of reaction, but 

 it should be stated again at the beginning of a discussion of genie 

 control of development. 



In its most general aspect, animal development consists of an or- 

 derly, seriated group of determinations. This means a continuous nar- 

 rowing dowTi of prospective potencies. In the beginning, the egg is 

 omnipotent. After some time ( which might begin with fertilization or, 

 in predetermined eggs, with oocyte growth), right-left halves are de- 

 termined; afterward, the location of some primordia for entire groups 

 of organs; then, of individual organs and their parts. The classic 

 methods of experimental embryology have firmly established this se- 

 quence of hierarchical steps of successively narrowing determinations 

 of embryonic material, sometimes by abrupt diversification, sometimes 

 (possibly always?) by an intermediate period of labile determination. 

 Genie action is in control of an efiBciently timed and spaced series 

 of determinations, followed by specific differentiation. The latter point 

 is of importance. Determination takes place in undiflFerentiated or 

 little differentiated cells and groups of cells, both in animals and in 

 plants. Visible differentiation follows final determination. The latter 

 fact opens the possibility of different behavior of these phases in 

 regard to genie control. Still another process of embryonic differentia- 

 tion may play a role, a process which is not directly provided for by 

 genie control. This is embryonic regulation and integration, which 

 means that in an experimental or organically controlled ( by a mutant ) 

 upset of normal development, the embryo is able to regulate and 

 integrate itself into a more or less perfect whole by means of processes 

 which were not provided for in the normal functioning of the genie 

 material. We shall return to this most important fact. 



In order to understand genie action in controlling development, 

 we must find out what the genie material does to control growth, 

 diversification of cell material in regard to prospective potency (i.e., 

 determination and intracellular specific tissue differentiation), the 

 proper timing of these events in relation to each other, the proper 

 spatial arrangement, and the chemodifferentiation. As the actual 

 processes of development take place within the cytoplasm of the cell, 

 additional problems arise as to whether the nucleus remains constant 

 while it controls cytoplasmic diversification or whether the genie 



