CERTAIN GENERAL CHARACTERISTICS 55 



largely of growth in size, growth gradients appear in many parts as either 

 absolute or relative differences in growth rate. Huxley's recent quantita- 

 tive study of relative growth is of interest in this connection (Huxley, 

 1932).^° Growth gradients and allometric growth occur very generally in 

 the course of development. Allometric growth, that is, growth of a part at 

 a different rate from growth of the whole, appears almost universally in 

 the apicobasal or longitudinal axis at some stage of development. In the 

 hydroids Tuhularia and Corymorpha length of hydranth in early stages 

 may be half or more than half the total body length, but both absolute 

 and relative lengths of stem increase until in large individuals stem length 

 is many times hydranth length. Evidently the stem grows in length more 

 rapidly than the whole. In planarians also, relative growth in length with 

 increase in total length increases from anterior to posterior levels (see, 

 e.g., Abeloos, 1928). Embryologically the pharynx arises near the pos- 

 terior end, but in long individuals it lies anterior to the middle. This 

 higher growth rate of posterior regions is most conspicuous in those 

 species which develop a posterior zooid or zooids and undergo fission; but 

 in these forms it is not, properly speaking, growth of a single individual. 

 Such growth gradients do not necessarily correspond in direction with the 

 primary gradient of the longitudinal axis, and certain characteristics of the 

 primary gradient may persist independently of growth gradients of later 

 stages. In Tuhularia and Corymorpha the high end of the gradient, indi- 

 cated by respiration, differential susceptibility, and reduction of vital 

 dyes, is the hydranth; but it is the low end of the gradient of longitudinal 

 growth in postembryonic stages. In fact, growth is very often greater at 

 the lower than at the higher levels of gradients, indicated by differences 

 in respiration and various other physiological factors.^^ In general, the 

 correspondence between these gradients indicated by different methods 

 appears to be closer in earlier than in later stages of development. Un- 

 doubtedly, gradients characterized by other activities than growth occur. 

 In fact, higher growth rate at lower levels of such gradients may occur 

 because other activities are less intense there. 



Graded morphological differences corresponding to the physiological 

 gradients present often appear, particularly in the longitudinal axes, in 

 adult animals. Distribution of setae, pigmentation, and nephridia of cer- 



^^ Huxley and Teissier (1936) have suggested that the term "heterogony," used by Huxley 

 (1932), be replaced by the term "allometry." 



^' In his discussion of gradients Needham (1931, pp. 582-606) has apparently failed to 

 grasp this point, that a gradient indicated by differences in respiration, susceptibility, etc., 

 does not necessarily correspond to a growth gradient in the same axis. 



