PATTERNS OF DEVELOPMENTAL PROGRESS 721 



asymmetrically, or axially from regions of highest rate of growth, the 

 growth centers. Adventitious buds of plants and buds in many animals 

 are, in their earliest stages, radial growth-gradient systems (see Figs. 1-4) 

 and become axial, heteropolar gradient systems in consequence of differ- 

 ential growth. Many organs — tentacles of most coelenterates, bryozoa, 

 and other forms; arthropod appendages; vertebrate limbs; etc.— begin 

 their growth as budlike systems, but many of them are not strictly radial. 

 The hydra bud shows a differential in tentacle development, apparently 

 in relation to the longitudinal gradient of the parent body (pp. 634-35). 

 In the amphibian limb an anteroposterior, and later a dorsiventral, differ- 

 ential, both influencing later growth, are determined in relation to the 

 general body axes; but the longitudinal axis of the limb is the result of 

 differential growth in the asymmetrically radial primordium. Even if 

 there is no growth of the whole in early embryonic development, for ex- 

 ample, during cleavage, there is usually more or less differential growth, 

 the regions about the apical pole growing more rapidly than, or at the 

 expense of, other regions. The growth-gradient systems of early stages 

 usually, if not always, coincide with the gradient systems otherwise in- 

 dicated or demonstrated. Primordia of organ systems and organs usually 

 become evident to the eye as growth-gradient systems. Various organ 

 primordia of the chick embryo in very early stages are characterized not 

 only by growth-gradient systems but by dye-reduction gradients in low 

 oxygen after staining, the two being coincident, as far as can be deter- 

 mined. The growth-gradient system of the anal arm of the sea-urchin 

 pluteus can be completely obliterated by inhibiting agents; and when it 

 is so obliterated, neither dye-reduction nor susceptibility gradients ap- 

 pear. 



In the course of development the boundaries of organs and organ sys- 

 tems usually become more definite, and growth of these organs and sys- 

 tems is correspondingly limited; but within those limits growth gradients 

 may still be present, and the body as a whole may also show growth 

 gradients in these later stages. These are by no means always simple and 

 often change in form with time. Data and graphs for many such gradients 

 have been given by Huxley (1932, chap. iii). In some crustacean append- 

 ages there is a decrease in both directions from a growth center at a cer- 

 tain level. Also, the steepness of the gradients and the position of the 

 center may change, and new centers may appear in some of these ap- 

 pendages. As development progresses, the growth gradients become in- 

 creasingly species-specific in their spatial characteristics, though they may 



