78 CM. CHILD 



4. If the group of cells which gives rise to the embryonic 

 tissue at the anterior end of the piece is designated x, the similar 

 group at the posterior end, z, and the remainder of the piece, 

 y, we may express head-frequency in pieces in a very simple 



rate x 

 form, viz.: head-frequency = . '. Tail-frequency, on the 



rate y 



other hand, is directly proportional to rate y. 



5. In pieces of considerable length the new head is localized 

 at the anterior end of the piece because the axial gradient deter- 

 mines that the cells at this end are physiologically isolated to a 

 much higher degree than the cells at the posterior end. A group 

 of cells developing independently at a transverse cut surface gives 

 rise to a head, but when developing in subordination to other 

 parts, gives rise to a posterior end. In short pieces, biaxial 

 heads, biaxial tails or reversal of polarity may occur according 

 to the relations between the rates of the regions x, y and z. 



6. The only logical conclusion from the data of experiment 

 and observation is that a single fundamental reaction system 

 is the basis of development and inheritance in each species, race 

 or individual. The apical or head region, or the dominant part 

 of that region, represents the fundamental reaction more nearly 

 than any other part of the organism. 



