66 INDIVIDUALITY IN ORGANISMS 



At best these various kinds of gradients are merely 

 general indications of differences in metabolic rate, 

 and undoubtedly in many cases the visible differences 

 along an axis represent something more than differences 

 in metabolic rate. The important point is that visible 

 indications of graded differences in metabolic rate occur 

 so generally in definite relations to the chief axes of the 

 body. 



In the animal egg a gradient in the distribution of 

 the yolk is often visible before development begins, and 

 in such cases that part of the egg which gives rise to the 

 apical region of the embryo contains less yolk than the 

 basal region. Associated with this gradient in most 

 cases we find differences in the size of cells appearing 

 in very early embryonic stages. In the egg of the frog, 

 which is an excellent example of this sort of egg, the 

 yolk gradient is very distinct, and the early develop- 

 mental stages show a gradient in the same direction in 

 the rate of cell division and the size of the cells formed 

 (Figs. lo, ii). The yolk gradient and the associated 

 gradient in cell division differ widely in different kinds of 

 eggs: in some cases only the apical region of the egg 

 divides at all, other parts serving as a source of nutrition 

 which is gradually used up during development. At 

 the other extreme are cases in which no yolk gradient is 

 distinguishable and differences in division rate and size 

 of cells do not become evident until later stages. 



In all cases developmental gradients of some sort 

 appear sooner or later as expressions of the metabolic 

 axial gradients and usually become more distinct as 

 morphological development proceeds. The so-called 

 law of antero-posterior development is a partial recogni- 



