PRINCIPLES OF VERTEBRATE MORPHOLOGY 11 



The Inhibiting Influence of Dominant Regions Upon the De- 

 velopment and Degree of Differentiation of Subordinate Regions. 



Among the invertebrates that have the same three fundamental 

 axial gradients possessed by vertebrates, experiments have shown that 

 during development a dominant region exercises an inhibiting in- 

 fluence over a subordinate region. This may be demonstrated by 

 experiments in regeneration. If a planarian worm is cut transversely 

 across the axis of polarity at almost any level the posterior piece will 

 grow a new head. Evidently then, while the posterior part was or- 

 ganically connected with the anterior piece, head formation was in- 

 hibited. As soon as the organic connection with the original head or 

 dominant part has been severed, the inhibition is removed and a head 

 develops. In another flatworm, Microstomum, the dominance of the 

 head becomes less with age so that at a point about halfway down the 

 axis new brain and eyes form. This means that a new head is estab- 

 lished, but there is not an immediate isolation of the new individual 

 from the old. As long as the new individual remains a part of the old, 

 it remains a subordinate individual, not only functionally in following 

 the lead of the original head, but structurally in that the eyes do not 

 fully differentiate and the brain remains small. Each of the two 

 individuals divides again into two and in each case the posterior 

 individual is subordinate to the anterior. 



In general therefore it would appear that the presence of a dominant 

 part inhibits the full differentiation of subordinate parts in proportion 

 to their relative metabolic rates or intensities and the proximity of the two 

 regions. 



Let us now apply these principles to vertebrate development. In 

 the first place it will be recalled that vertebrates are metameric 

 animals in which each metamere, at least back of the primitive head, 

 is serially homologous with all the rest. Each metamere has therefore 

 potentially the developmental capacity of any other. Whatever sys- 

 tems or structures that develop in any one metamere should, ex 

 hypothese, be latent in all of the others. Any failure, therefore, on the 

 part of a given metamere to realize the full extent of its differentia- 

 tional possibilities must be attributed to some sort of inhibition. Let 

 us examine the main regions of the body of a higher vertebrate with 

 this idea in mind. 



The head metameres are characterized by a great specialization of 

 brain tissue and an almost complete suppression of the lower functions. 



