PROBLEMS AND MATERIAL 9 



will produce the same substances. Dominance of this kind unquestionably 

 occurs widely in organisms but evidently increases in complexity and im- 

 portance with increasing differentiation of organs, both in individual de- 

 velopment and in evolution. The hormone interrelations in the higher 

 vertebrates represent its highest development. Transmissive dominance 

 is a control based primarily on intensity factors; transportative domi- 

 nance, on specific substances. The one may be regarded as essentially 

 quantitative; the other, as qualitative. Both transmission and transport 

 may be concerned in some types of control, as, for example, in the media- 

 tion of nerve impulses by chemical substances. According to current the- 

 ory, movement of ions occurs in transmission of excitation, but only over 

 short distances; and transmission depends on the electric charges, not on 

 the chemical nature of the ions. The problem of physiological integration 

 of the organism as a whole is the problem of the origin and nature of the 

 factors concerned in dominance and of their effects on subordinate parts. 

 Self-differentiation or independent difTerentiation of parts, correlative or 

 dependent differentiation, induction, organizers — all raise questions con- 

 cerning the part which physiological dominance plays in development and 

 the nature of the factors concerned in each case. From another viewpoint, 

 the nature of excitation and transmission, the methods of transport of 

 substances, and the chemical constitution of hormones and other trans- 

 ported substances and of the parts affected are all involved in the problem 

 of physiological integration. 



In consequence of the researches of Spemann and his co-workers on 

 amphibian development the terms "inductor" and "organizer" have found 

 wide acceptance among students of developmental physiology. The con- 

 cepts of inductor and organizer have undergone changes as amphibian ex- 

 perimentation has advanced. The two terms have often been used indis- 

 criminately, but with progress of experiment distinction has become in- 

 creasingly desirable. At present an "inductor" may be defined as an agent 

 which brings about a definite developmental effect — a determination or 

 differentiation of a particular tissue, for example, or a new axiate pattern. 

 In earlier amphibian experiment the inductors were parts of a developing 

 embryo, either of the same species as that in which the induced effect 

 occurred or of another species. An organizer is an inductor which deter- 

 mines a definite, orderly developmental pattern in another part. The in- 

 ductors from the region constituting the dorsal lip of the amphibian blas- 

 topore are, in general, also organizers, since they determine a new axiate 

 pattern, an organization. As experiment progressed, it was discovered 



