PRINCIPLES OF VERTEBRATE MORPHOLOGY 9 



animal structure that has yet appeared. His " axial gradient theory'' 

 is a statement in energistic terms of the axiate organization of animals 

 and of the relations of dominance and subordination that are so ob- 

 vious in such animals as the vertebrates. He has shown experimen- 

 tally "that the apical or anterior region is primarily the region of 

 greatest dynamic or metabolic activity in the individual. The apical 

 end becomes the most highly specialized and differentiated region of 

 the body, and in those forms which possess a central nervous system, 

 the cephalic ganglion or brain and the chief sense organs usually 

 arise in this region; ... in other words it becomes the head, and in 

 motile forms usually precedes in locomotion." "The basal or post- 

 erior region, on the other hand, is primarily the least active region and 

 in motile forms its activity is more or less under the control of the 

 apical or anterior region." 



THE AXIAL GRADIENT IN DEVELOPMENT 



There is a remarkable parallelism between the orderly spatial 

 arrangement of structures and functions down the polar axis and the 

 orderly sequence in time of these structures during the develop- 

 ment of the individual from the egg. The first structures to differ- 

 entiate are those of the head; in fact the early vertebrate embryo is 

 nearly all head and the body grows out from it as though it were a 

 mere axial outgrowth of the head. The last part of the body to com- 

 plete its differentiation is the tail or basal part of the axis of polarity. 

 There is thus an important relation between the degree of dominance 

 and the developmental age of the various levels along the axis. The 

 more anterior part always differentiates before the more posterior 

 part and dominates it at least for a time. Secondary alterations in the 

 relationships of dominance and subordination are of frequent occur- 

 rence and result from functional adjustments and through the action 

 of external factors. The same parallelism between spatial arrange- 

 ment and sequence in time exists for the dorso-ventral axis. In verte- 

 brates, the first structures to differentiate are those in the medullary 

 plate region; in fact, in the higher vertebrates, the early embryo con- 

 sists almost entirely of the anterior median dorsal region, which is 

 destined to form the central nervous system of the head. The last 

 structures fully to differentiate are the reproductive structures which, 

 functionally at least, belong to the basal part of the gradient. Struc- 

 tures also develop mesio-laterally, differentiating first in the middle 



