§1.2 CHEMICAL ACTIVATORS 3 



B. True Activators. Chemical substances produced by the 

 organism and exerting specific functions in regard to corre- 

 lation or differentiation: 



1. Local activators, with effects on the same cell, or cells, within 

 which they are produced. 



(a) Intracellular activators ("intracellular hormones" of 

 Goldschmidt), acting in each cell singly and being the 

 direct expression oi gene activity, in relation to regional 

 differentiation. 



(b) Regional activators, responsible for the chemodijferentia- 

 tion of specific regions in embryos and for growth 

 gradients. 



2. Distance activators, with effects on cells other than those in 

 which they are produced. 



(a) Diffusion activators, distributed by diffusion through the 

 tissues. 



(i) Direction of transport restricted by structural 

 organization. Growth hormones in plants. 



(ii) Diffusion restricted to tissues in direct contact. 

 Organizers in embryos and ^'organisines'^ in animals 

 without a circulatory system. It is possible that the 

 cortical releasing factor, CRF, from the brain of 

 vertebrates should also be included here (§ 4.323). 

 (iii) Diffusion restricted mainly by chemical means. 

 Neurohumoral secretions at nerve- and neuro- 

 secretory cell-endings ("neurohormones" of Welsh, 

 1955). 

 (iv) Diffusion unrestricted, the substances passing out 

 of the tissues and into the surrounding medium to 

 act on other individuals, usually of opposite sex. 

 These include ''gamones'' and ''ectohormones'\ 



(b) Circulatory activators or vascular hormones, distributed 

 to all parts of the body in the blood circulation, so that 

 their actions must be limited by the sensitivity or com- 

 petence of the tissues which they reach. They may be 

 secreted by: 



(i) Isolated cells such as those of unknown origin in the 

 gut mucosa of vertebrates (§ 2.21). 



