PROBLEMS OF ORGANIZATION 197 



differentiated into chorda, somites, gut and mesonephric ducts ; in this 

 case, however, the neural structures induced from the accompanying 

 ectoderm were spinal rather than cranial in character. The three por- 

 tions of the archenteric roof are thus demonstrated to be essentially 

 similar to the prechordal plate, the head organizer, and the trunk 

 organizer respectively of the amphibian gastrula (cf. E. K. Hall ('37) ). 



In fish organization before gastrulation the action of the different 

 components is more tentative than fixed, and while the tentative reac- 

 tions have certain definite sequelae which lead to the formation of 

 organized materials, they themselves are not irrevocably placed in posi- 

 tion where they cannot have different attributes from those which nor- 

 mally develop from them. During gastrulation there is a much more 

 specific organization of the blastoderm as a whole. The essential nature 

 of organizing forces operating in the integration of the developmental 

 processes remains to be elucidated by methods not yet at our command. 

 But in teleosts, where the formation of the embryo is limited to a rela- 

 tively small portion of blastoderm, there are demonstrable integrated 

 activities w^hose role is the coordination of the various parts of the 

 blastoderm. There is evidence both in Salmo and in Fundulus which 

 suggests the existence of such activity and which indicates that their 

 nature is not identical in the two forms. 



The various parts of the blastoderm have different relationships with 

 one another. Luther ('36) divided or sectioned the blastoderm into six 

 sectors and found that not all parts of the blastoderm are equivalent in 

 embryo formation. At all stages investigated the potencies for differen- 

 tiation as indicated by the percentage of grafts differentiating were 

 highest in the embryonic sector and lowest in the lateral portions of the 

 blastoderm, diminishing gradually around the blastoderm during the 

 course of gastrulation. The differences between the median embryonic 

 and more outlying regions become progressively greater. No differentia- 

 tion was attained when a central portion of the blastoderm was isolated. 

 Luther interprets the results of these isolation experiments as demon- 

 strating the existence of w^hat he called a physiological field. He postu- 

 lated that the potency for differentiation in one particular sector was 

 related to the state of physiological activity of its constituent cells, which 

 in turn varies according to the position of the cells with respect to the 

 whole and according to their age. It is highest in the embryonic sector 

 and lowest in the extraembryonic. In the lateral embryonic and in the 

 extraembryonic regions it diminishes progressively during the course 

 of gastrulation. Luther ('37) considered his experiments both by 



