54 PATTERNS AND PROBLEMS OF DEVELOPMENT 



gradients in rate of cell division, in size of cells, and in rate of growth and 

 of differentiation appear in relation to the axes of the whole and of parts 

 as development progresses. 



The very general occurrence of an anteroposterior developmental 

 gradient has been recognized in the so-called "law of anteroposterior, 

 cephalocaudal, or craniocaudal development.'"^ But developmental gra- 

 dients equally definite in character and equally constant in occurrence 

 appear in other directions — for example, from the ventral region laterally 

 and dorsally in turbellarians, annelids, and arthropods and from the mid- 

 dorsal region laterally and ventrally in vertebrates. In certain other 

 groups — coelenterates, trematodes, cestodes, nemerteans, nemathel- 

 minths, mollusks, and echinoderms — an apicobasal or longitudinal de- 

 velopmental gradient appears more or less clearly, at least in the egg, in 

 cleavage or in larval stages; but other gradients differ according to de- 

 velopmental pattern in different members of these groups. As develop- 

 ment proceeds, new gradients may arise at various levels and in various 

 directions with localization of organ primordia. Every such localization 

 alters, and may even obHterate or reverse, the pre-existing gradient or 

 gradients. Development of the first hydranth from the originally basal 

 end of the planula of calyptoblast hydroids with reversal of the original 

 polarity is an example (pp. 96, 97). Many axiate organs and organ sys- 

 tems make their appearance as localized areas or fields of developmental 

 activity in which the activity decreases from a region more or less cen- 

 trally or otherwise localized in the area. A tentacle, for example, usually 

 begins its development as a region in which developmental activity de- 

 creases more or less radially from a center and becomes axiate in conse- 

 quence of greater growth of the central region ; its developmental pattern 

 is essentially like that of other buds. Conditions in early developmental 

 stages of many appendages are apparently similar, but the most active 

 region is not necessarily at the geometrical center of the field. In short, 

 observation alone gives evidence of gradients and gradient systems ap- 

 pearing and undergoing change in definite order and relation as char- 

 acteristic features of earlier embryonic developmental stages of many 

 organisms and organs. Gradients appear to precede sharply defined and 

 bounded morphogenetic differentiation. 



Even in advanced stages of development, in which the changes consist 



'9 Morphologically, this gradient is merely a timetable of events, but there must be a 

 physiological basis for this timetable. In this connection see Kingsbury, IQ24, 1926, 1932; 

 Child, 1925c. 



