DIFFERENTIAL DEVELOPMENTAL MODIFICATION. Ill 271 



fective from the beginning of development to maturity in many of the 

 simpler organisms and in some of the organ systems, even of vertebrates 

 and man. Such factors become evident, for example, in the reconstitution 

 of isolated pieces of hydroids, planarians, and many other forms; the 

 localization of the reconstituting hydranth or head evidently depends on 

 these gradient factors rather than on the specifically differentiated organs 

 in the piece. Gradient factors are also evident in the functional gradients 

 of many axiate organs — for example, the mammalian intestine (see 

 p. 164). 



In many animal eggs specific regional differentiations and metaboHsms 

 are apparently already present in the cytoplasm at the beginning of em- 

 bryonic development, but quantitative gradient factors may also be pres- 

 ent and effective. All that we know concerning developmental physiology 

 indicates progressive increase of specificity of parts during the earUer 

 stages: the progress of so-called "determination of parts" and of visible 

 differentiation certainly constitute abundant evidence of this increase. 

 By a sort of physiological extrapolation to successively earlier stages of 

 development, to the ovarian development of the oocyte, to the beginnings 

 of reconstitution in the isolated piece, to the bud in its initial stages, and 

 to the aggregate of dissociated cells we seem to find it necessary to postu- 

 late a quantitative gradient pattern as the primary axiate pattern, and 

 many fines of experiment show the presence of such pattern. It is quite 

 unnecessary, however, to assume that every individual organism begins 

 its development with nothing but this primary gradient pattern. Obvi- 

 ously, this is not the case. The reconstituting planarian piece may con- 

 tain various organs; its polarity usually determines on which end the 

 head shah develop, and the ventrodorsality of the regenerated head is evi- 

 dently derived from that of the piece, but the new gradient pattern re- 

 sulting from activation and formation of new growing tissue at the an- 

 terior end makes over — -reorganizes — the piece. AH that we can learn con- 

 cerning the organization of eggs at the beginning of embryonic develop- 

 ment and the changes during the course of development is, of course, of 

 value in the analytic investigation of that kind of development; but it 

 should not be forgotten that this organization may be far from the pri- 

 mary developmental pattern and significant rather as a feature of a par- 

 ticular kind and stage of development than as representing fundamental 

 factors of developmental pattern. We have already learned that various 

 features of egg organization are results — effects — rather than essential 

 factors of the real pattern. 



