154 c - M - CHILD. 



pears more or less clearly. In at least most bilateral invertebrates 

 differentiation of the body-wall and its organs progresses from 

 the median ventral region laterally and dorsally (Child, 'i5c, Fig. 

 13) and in vertebrates from the median dorsal region laterally 

 and ventrally, as a moment's examination of the chick embryo 

 shows. The so-called law of antero-posterior development is 

 merely a recognition of the existence of this developmental 'gra- 

 dient in the longitudinal axis, and it is, moreover, only a partial 

 statement of what might be called the law of axiate development. 

 We also find evidences of the existence of a polar axial gradient 

 in the rate of regulatory development and the position and pro- 

 portions of organs in isolated pieces from different levels of the 

 body in various forms (Child, 'ojb, 'o?c, 'lib; Hyman, '16). 



The Evidence from Susceptibility. The existence of the axial 

 gradients is perhaps most readily demonstrated through the sus- 

 ceptibility of organisms to the action of various external agents. 

 It has been determined experimentally for many species of ani- 

 mals including all the chief phyla and many of the smaller groups 

 and nearly a hundred species of algae among plants that axial 

 gradients in susceptibility to the action of at least a wide range of 

 external agents exists. The agents used in these experiments 

 include cyanides, many anesthetics such as alcohol, ether, chloro- 

 form, chloretone, some of the urethanes, etc., carbon dioxide and 

 various acids, alkalies, neutral salts, certain alkaloids, vital 

 dyes and physical conditions such as extremes of temperature 

 and the negative factor lack of oxygen. 1 Potassium cyanide has 

 been much used in these experiments first because very low con- 

 centrations are effective and second, because it has been found 

 by a large number of investigators to be a powerful inhibitor of 

 protoplasmic oxidation, and susceptibility to cyanide may there- 



i The data on susceptibility as determined by death and disintegration of 

 cells or tissues, so far as they have been published, appear in the following 

 papers: algae, Child, 'i6c, e, '170, b, '200,, protozoa, Child, '14^, Hyman, '17; 

 ccelenterates, Child, '18, 'igb, Child and Hyman, '19; ctenophore, Child, '\-jc; 

 Planaria, Child, '12, '130, 'i3&, '140, d, 'i6b, 'igc, d; echinoderm eggs and em- 

 bryos, Child, '150, 'i6a; annelids, Child, 'i^d, Hyman, '16; amphibia, Bellamy, 

 '19; miscellaneous, Child, '140, '150, pp. 50-62. Further data on susceptibility 

 gradients in protozoa, ctenophores, hydrozoa, flatworms, echinoderms, anne- 

 lids, fishes and amphibia obtained by Bellamy, J. W. MacArthur, Hyman and 

 Child are not yet published. 



