344 PATTERNS AND PROBLEMS OF DEVELOPMENT 



natural and experimental environmental conditions. Proportional scale 

 of different pattern components may also show similar variation (sec 

 chaps, v-vii). Reconstitution of parts of eggs, of isolated blastomeres 

 or blastomere groups, and of parts of later embryonic stages shows in 

 some forms considerable variation in scale of organization with size of 

 isolated part; but experiments directed toward alteration and control of 

 scale are few. In many eggs scale is so stably determined at the beginning 

 of embryonic development that no great alteration appears under condi- 

 tions thus far employed. Although data of many authors show differences 

 in scale, their importance for problems of embryonic, as well as reconstitu- 

 tional, development seems not to have been fully recognized. 



At present it appears difficult to account for many of the expressions 

 of scale of organization except in terms of dominance and gradient pat- 

 tern. According to these terms, a gradient established in an isolated part 

 in good condition will, in general, represent a more intense activity at its 

 high end, will extend over a greater distance, and will perhaps be less 

 steep than one established under unfavorable conditions; consequently, 

 the local fields developing along its course, within which the particular 

 parts develop, will also be longer and the scale of organization therefore 

 larger. Such an interpretation must, of course, remain general until we 

 know more about metabolism and its changes at different levels of a 

 physiological axis in reconstitution and embryonic development and about 

 the relation of dominance and determination to such factors. A localized 

 active region, however it originates, may determine a gradient or gradient 

 system and so become a dominant region, or a gradient may arise as a 

 direct reaction to an environmental gradient. In the latter case the en- 

 vironmental gradient is merely the initiating factor; protoplasmic consti- 

 tution and condition are the factors determining the final character of the 

 gradient; and the high end apparently is more or less dominant. Domi- 

 nance and the gradient establish a physiological basis for definite axiate 

 pattern. In short, the dominant region is or may be an inductor or "or- 

 ganizer," that is, it determines the organization or reorganization of other 

 parts. A few examples of alteration of scale of organization in reconstitu- 

 tion will serve to indicate some of the relations between scale and certain 

 factors. 



SCALE OF ORGANIZATION IN HYDROID RECONSTITUTION 



Many investigators have noted the great variation in length of the 

 hydranth primordium in reconstitution of stem pieces of Tubularia.'' The 



" See, e.g., Bickford, 1894; Driesch, 1897, 1899; Morgan, igoib, 1902(7, 1903(1; Child, 1907a. 



