PATTERNS OF DEVELOPMENTAL PROGRESS 707 



reconstitution of isolated parts, by development of parts following trans- 

 plantation or explantation, by relation of development of various parts 

 to innervation, and by discovery of the role of chemical correlative fac- 

 tors, hormones, etc., that ordering, integrating, or correlating factors play 

 an essential part in development, that a physiological unity is essential 

 to initiation of orderly development. 



It was noted in chapter i that two groups of factors are concerned in 

 physiological integration — the transmissive or dynamic, consisting in 

 transmission of energy changes without mass transport of substance from 

 region of origin to region of effect, and the material, transportative, or 

 chemical factors, consisting in production by certain parts of the organism 

 of chemical substances and their mass transport by one means or another 

 and action on other parts. These factors constitute the physiological basis 

 of the unity of the organism, of the organism as a whole. 



Transmission of mechanical, thermal, or electrical energies is possible in 

 living protoplasms; but the transmissive factor most important physio- 

 logically is transmission of the physiological change known as ''excita- 

 tion." Difference in electric potential between regions or parts of an or- 

 ganism results from many differences in physiological condition; but the 

 fact that characteristic potential differences are maintained, even when 

 regions concerned are connected by a conductor, indicates that the chief 

 ultimate source of the differences is metabolism. There are grounds for 

 believing that electrical transmission resulting from local activation or 

 from regional differences in activity is the most general and primitive in- 

 tegrating factor on an organismic scale. According to current theory, elec- 

 tric transmission is an essential factor in conduction of the nerve impulse.^ 

 Granting this, it is doubtless also essential in the more primitive transmis- 

 sions of excitations or activations in protoplasms generally. The simplest, 

 most primitive sort of organismic or developmental pattern appears to 

 be the spread or transmission, usually or always with a decrement, of the 

 effect of a local activation or excitation of a protoplasm. Such an excita- 

 tion-transmission gradient is possible without any pre-existing local dif- 

 ferentiation of the regions concerned. In consequence of its activation the 

 region of primary excitation or activation becomes, for the time, a domi- 

 nant region. Various lines of experiment on determination of polarity 

 discussed in earlier chapters show that, if the activation persists, the 



' See, e.g., R. S. Lillie, 1922, 1923, 1936; Adrian, 1932; and citations by these authors. The 

 action currents associated with nervous conduction have been studied by a host of investi- 

 gators for many years. 



