PHYSIOLOGICAL GRADIENTS. 153 



axial gradients in all cases examined, though these organ gra- 

 dients do not necessarily coincide in direction with the primary 

 gradients. As was pointed out nearly five years ago (Child, 'i^c, 

 pp. 54, 60) the primary gradient or gradients may persist 

 throughout life, but do not necessarily do so. They may undergo 

 modification and complication in various ways during the course 

 of development : the originally quantitative relations may become 

 qualitative, new gradients may arise in certain parts or organs, 

 in some cases preexisting gradients may be broken up, obliterated 

 or reversed, one kind of axiate pattern may be replaced by an- 

 other, and so on. In all cases, however, the sequence of events 

 is definite and orderly and many features of it can already be 

 interpreted in physiological terms, and usually the pattern even 

 in the fully developed organism shows a very definite relation 

 to the primary gradients. The evidence for the existence of 

 metabolic or physiological axial gradients is varied and exten- 

 sive and only a brief summary is possible here. 



Structural and Developmental Gradients. First of all many 

 eggs and embryos show an apico-basal gradient in protoplasmic 

 structure and content, e.g., the gradient in yolk accumulation in 

 many animal eggs and embryos and the gradient in protoplasmic 

 density and vacuolation in many plant embryos (Child, '15^, 

 Figs. 8, 19) and in the vegetative axes of many of the simpler 

 plants. 



A gradient in the rate of cell division, growth and differentia- 

 tion in relation first of all to the primary or apico-basal axis, later 

 in relation to other axes, is a very general feature of at least the 

 earlier stages of development. This gradient appears first in 

 the rate of cell division and size of cells along the apico-basal 

 axis in a large proportion of both animal and plant eggs (Child, 

 'i5c, Figs. 10, n, 18, 19) and in many plant axes, as a gradient 

 in rate of division and cell size from the growing tip basipetally 

 (Child, 'i^c, Figs. 20, 21, 22, 36-39). It also appears in the 

 progress of morphogenesis and differentiation along the axes 

 particularly in animals, although in plants, except for the fact 

 that the growing tip itself remains embryonic, the course of dif- 

 ferentiation is also in general basipetal. Moreover, in bilateral 

 animals and plants a similar bilateral developmental gradient ap- 



