72 INDIVIDUALITY IN ORGANISMS 



Fig. 17 is a transverse section at a stage of development 

 corresponding to that attained at the level of the sixth 

 somite of the embryo in Fig. 15. At this stage the 

 embryonic nervous system is present in the form of a 

 tube open dorsally, and differentiation has progressed 

 both laterally and ventrally from the median dorsal 

 region. In the other vertebrates, including the mam- 

 mals, the developmental gradients are similar. 



Differences in rate of growth constitute another 

 feature of these developmental gradients, but the rela- 

 tion between the axial metabolic gradient and rate of 

 growth is not simple, for the period of highest growth 

 rate occurs at different times in different parts accord- 

 ing to the time of their formation, and it may happen 

 at certain stages of development that the rate of growth 

 at the apical end of a metabolic gradient is lower than at 

 the basal, because the region at the apical end began 

 its growth first, has grown at a more rapid rate, and is 

 therefore completing its growth earlier than the region 

 at the basal end. Nevertheless, so far as it is possible 

 to compare corresponding stages in the development of 

 different parts, along an axial gradient, differences in 

 rate of growth corresponding to the gradient do appear. 

 The head-region, for example, at the stage of highest 

 growth rate grows more rapidly than the posterior 

 region of the body at its stage of highest rate, and 

 similar relations exist with reference to other gradients. 



In the egg of the plant as well as in that of the animal 

 developmental gradients usually appear in early stages. 

 In the eggs of many of the lower plants the first division 

 is transverse, the two cells thus formed representing 

 apical and basal regions of the plant, and in most of the 



