40 PROTOPLASMIC ACTION AND NERVOUS ACTION 



anaesthetics, cyanide, cold) have a similar effect/ 

 These substances hinder or suppress the growth of the 

 anterior region of the forebrain between the optic 

 vesicles so that the latter tend to approximate and 

 coalesce, producing a single instead of a double structure.^ 

 The production of exogastralae from sea-urchin blastulae 

 by adding lithium chloride to the sea water is a similar 

 instance; in this case the endoderm grows outward 

 instead of inward.^ The transition is direct from such 

 simple cases of artificial chemical control of development 

 to the cases where various developmental processes occur 

 normally under the control of special chemical substances 

 produced by the organism itself; the influence of hor- 

 mones illustrates such^ cases; metamorphosis (in tad- 

 poles), the growth of the skeleton, and the production 

 of sexual characters are thus determined. 



In general any condition affecting the rate or character 

 of the formative metabolic reactions has a corresponding 

 influence on growth and development. Such conditions 

 include the influence of physical agents like electricity, 

 light, temperature, contact. It is significant that the 

 term ''irritability" is applied, especially in plant physi- 

 ology, to the susceptibility of growth processes to such 

 modifying influences; in such cases the organism ''re- 

 sponds" by changing its rate or manner of growth. Any 

 such response implies a corresponding modification in con- 

 structive metabolism; hence such facts show that re- 



^ Cf. Stockard, American Journal of Anatomy, X (1910), 369; 

 McClendon, American Journal of Physiology, XXIX (191 2), 289. 



' Cf. the discussion in Child's Origin and Development of the Nervous 

 System, pp. 36 flf. 



3 Cf . Herbst, Z. wiss. Zool., IV (1892), 446; Mitteilungen zool. 

 Sta. Neapel, XI (1893), 136. 



