724 PATTERNS AND PROBLEMS OF DEVELOPMENT 



development with an agent having high differential action, such as LiCl, 

 give forms resembling sea-urchin gastrulae. With less extreme differential 

 action starfish larvae approaching crinoid larvae in form can be pro- 

 duced. With secondary modification of gradient pattern by tolerance, 

 conditioning, or recovery, echinid larvae showing some approach to the 

 asteroid larval form result. These experimental modifications suggest that 

 evolution of echinoderm larval form has resulted in part from changes in 

 gradient pattern in consequence of change in specific constitution of the 

 protoplasm. 



The various head forms resulting from mediolateral differential inhi- 

 bition in planarian reconstitution, constituting a continuous series from 

 normal to acephalic, with obliteration of parts progressing laterally from 

 the median region (pp. 177-96), and the essentially similar results of 

 differential inhibition in vertebrate development (chap, vii) provide fur- 

 ther illustrations of the dependence of forms and proportions on physio- 

 logical condition of the protoplasm. The experimental alterations of scale 

 of organization (chap, x) show another aspect of this dependence. In all 

 these and many other developmental modifications the gradient pattern 

 has been altered differentially, with resulting alterations in rates and 

 amounts of growth and in localization, degree, or even presence or ab- 

 sence of certain differentiations. Since these differential modifications of 

 growth and form appear to be results of primarily quantitative differential 

 alterations of gradient pattern, they suggest the possibility that at least 

 some of the characteristic differences of growth and form in related species 

 may also be results of similar alterations, primarily quantitative, rather 

 than specific in character, so far as gradient patterns are concerned. Length 

 and steepness of gradients, rates and amounts of growth, and localizations 

 of new growth centers may be altered by genetic changes in protoplasmic 

 constitution, as well as by environmental conditions. 



By the use of Cartesian co-ordinates Thompson (191 7, chap, xvii) has 

 made evident graphically some of the changes in distribution of growth 

 in the bodies and in various parts of related species. The procedure con- 

 sists in applying a network or grid of rectangular co-ordinates to the out- 

 line of the body or part concerned in one species and deforming or trans- 

 forming the co-ordinate grid in a regular manner so that it becomes or 

 approximates a corresponding co-ordinate system for the body or part 

 to be compared with the first. The deformation or transformation re- 

 quired in the second case indicates, in a general way, when compared with 

 the first, the differences in growth distribution between the two in certain 



