28 THE ORIGIN OF THE NERVOUS SYSTEM 



in general basipetal. Moreover, in bilateral animals 

 and plants a similar bilateral developmental gradient 

 appears 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 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 anteroposterior development is 

 merely a recognition of the existence of this develop- 

 mental gradient in the longitudinal axis and it is, more- 

 over, 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, the positions and proportions 

 of organs in isolated pieces from different levels of the 

 body in various forms (Child, 19076, c, ignb; Hyman, 

 1916). 1 



THE EVIDENCE FROM SUSCEPTIBILITY 



The existence of these gradients is perhaps most 

 readily demonstrated through the susceptibility of 

 organisms to the action of various external agents. It 

 has been determined experimentally for many species 



1 Figures of these structural and developmental gradients are given 

 in Individuality in Organisms. Figures showing gradients in cell size, 

 rate of division, and in some cases in protoplasmic structure are as 

 follows: cleavage stages of the frog's egg, Figs. 10, n; early develop- 

 mental stages of plants, Figs. 18, 19; growing tips and axes of plants, 

 Fig;. 20-36; adventitious buds in plants, Figs. 37-39. In chaps, iv 

 and v of the Individuality some of the evidence for the existence of the 

 gradients from regulatory development and its experimental modifica- 

 tions is given. See also Figs. 13-17, of flat worm and chick on pp. 109 

 and 1 1 1 below. 



