DIFFERENTIAL DEVELOPMENTAL MODIFICATION. Ill 257 



may occur without appreciable brain reduction. Stockard's interpretation 

 of these modifications as indicating a median origin of the optic primordia 

 and prevention of their later separation or of attainment of the usual 

 position does not necessarily follow from the experimental data (see 

 pp. 282-85). 



In many of the modifications in fishes the head region is strongly in- 

 hibited; posterior regions, little or not at all, because exposure to the 

 inhibiting agent was temporary and ended before the posterior region 

 developed high susceptibihty. In various other cases further experiment 

 is necessary to determine whether differential conditioning or recovery 

 may be concerned in the result. In the more extreme degrees of inhibition 

 posterior regions are usually more or less inhibited or absent (Fig. 103, 

 C, D). Development of the heart may also be differentially inhibited by 

 external agents, so that it remains tubular; and a periodic reversal in direc- 

 tion of beat has been observed in certain cases (Gowanlock, 1923). 



Asymmetric forms with a single eye on one side of the head have been 

 described frequently. Probably many of these result from unequal expo- 

 sure to the inhibiting agent or from unequal oxygen supply or unequal dif- 

 fusion of CO,. If the developing embryos lie undisturbed on the bottom 

 of a container or if floating eggs are in contact or aggregated in groups, 

 such differences may arise. When exposure to the agent begins in early 

 stages, difference in stage of the division cycle on the two sides of the 

 blastoderm, with resulting difference in susceptibility at the time when 

 the agent becomes effective, may determine later asymmetry. But what- 

 ever the factor concerned in a particular case, it seems to be incidental. 

 Evidences of differential acceleration of development with production of 

 megacephalic forms in Macropodus have been obtained by Gowanlock.^ 



EXPERIMENTAL MODIFICATIONS OF AMPHIBIAN DEVELOPMENT 



In the many studies of modification of amphibian development differ- 

 ent agents, concentrations, and intensities have been used, different de- 

 velopmental stages have been exposed and for different periods, and a 

 great variety of forms has resulted. Most of these show the characteris- 

 tics of differential inhibition and indicate the presence of a definite differ- 

 ential susceptibility pattern, that is, the modifications with different 

 agents give little or no evidence of regional specificity.'* 



5 Unpublished. See Child, 19246, p. 85, for figure. 



^ The following references illustrate, to some extent, the development of investigation in 

 this field: O. Hertwig, 1892, 1895, 1898; Gurwitsch, 1895; C. B. Wilson, 1897; Bataillon, 1901, 

 1904; Bohn, 1903; Morgan, 1903; Schaper, 1904; Jenkinson, 19066, 1911a; Levy, 1906; Bardeen, 



