156 PAUL WEISS 



the parent species, reflects the fact that the numbers and kinds of molecu- 

 lar key species available for differentiation are limited by the inherited, 

 hence, genetically controlled, initial endowment. It would seem prema- 

 ture, however, to speculate about the relation between our postulated 

 key species and genes. 



Phenomena of dominance and recessivity of cellular activities can 

 likewise be explained in terms of our hypothesis. Any molecular species 

 assuming a monopolistic surface position thereby gains "dominance" 

 over competitively inferior species which, being barred from the sur- 

 face, are in no position to express themselves effectively. 



Induction 



After this consideration of cellular responses in differentiation, some 

 comment should be given to the factors calling them forth in the organ- 

 ized play of development. In view of the fact that the problems of 

 "organization" are discussed by Dr. Nicholas in the next chapter, we 

 shall forego any but the most cursory treatment of the subject here. 



According to the testimony of experimental embryology, develop- 

 ment proceeds as follows. Cleavage subdivides the egg into blasto- 

 meres, each of which receives in addition to a nucleus a certain fraction 

 of the original cytoplasmic system. While the nuclei are essentially 

 equivalent, the cytoplasmic allotments vary, depending on the degree of 

 specific molecular segregation attained in the undivided egg. The role 

 of the most nearly stabilized part of the egg, namely its surface, must 

 be especially taken into account.^ Different blastomeres inherit different 

 parts of the original egg surface. This initial differential among egg 

 parts is then further elaborated and amplified by the subsequent reactive 

 transformations which the various cleaved-off cells undergo in response 

 to the organized conditions of the system which they constitute. 



Descriptively, these conditions have been referred to as "fields"; but 

 we shall not concern ourselves here with their character and operation. 

 We need only keep in mind that in the early phases of development any 

 area of the cleaved egg is endowed with dual capacities. On the one hand, 

 it is composed of a large number of individual cells which in many cases, 

 according to experimental evidence, are practically equivalent ("equipo- 

 tential") and hence interchangeable as far as their response faculties 

 are concerned. On the other hand, the totality of these cells, as a collec- 



^ The organizational role of the surface of the egg has been emphasized by several 

 embryologists (9, 21, 33). 



