314 H. H. NEWMAN. 



THE AXIAL GRADIENT THEORY AND THE MORPHOGENESIS OF 

 MONSTERS RESULTING FROM HETEROGENIC HYBRIDIZATION. 



The axial gradient in vertebrates is essentially a quantitative 

 one depending on the rate of metabolic activity. The rate along 

 the apico-basal axis is highest at the anterior or apical end and 

 lowest at the posterior or basal end. Paralleling the axial meta- 

 bolic gradient there exists a susceptibility gradient, in which 

 the apical end is most susceptible to agents that lower the vitality 

 or retard the rate of metabolism. Similarly the median dorsal 

 region is the most susceptible point of the bilateral or mesio- 

 lateral axis and the lateral part the least susceptible. Certain 

 points that are both apical and median will be the most suscepti- 

 ble of the whole system. The eyes are such structures developed 

 from a primitive mesio-dorsal-apical anlage, and are therefore 

 the most susceptible of all parts of the body. 



In animals of the high grade of complexity exhibited by verte- 

 brates there are to be distinguished several semi-independent 

 axiate systems. Undoubtedly the central nervous system and 

 its derivatives constitutes an axiate system by itself. Similarly 

 the circulatory system is axiate with the heart the apical point 

 and the veins and arteries basal. The skeleton with the fin 

 systems are also semi-independent axiate systems, with the 

 anterior parts more susceptible than the basal ones. This is 

 brought out in recovery cases in which the pectoral fins recover 

 while the pelvic do not. 



Whenever young fish embryos are subjected to any of the 

 types of inhibiting or retarding agents that have been previously 

 discussed we are able to classify the resulting abnormal forms into 

 categories (modifying somewhat the usage of Child) : (a) Forms 

 in which there has occurred an inhibition of anterior and of 

 median dorsal regions; (&) Forms in which these regions have 

 become acclimated or have recovered from an initial inhibition. 

 Curiously enough in chemical experiments those parts that are 

 most susceptible to inhibiting agents of higher concentrations are 

 able most completely to acclimate themselves in lower concen- 

 trations or to recover when the inhibiting agent is removed or 

 its severity lessened. Let us consider these two categories of 

 teratomata in some detail. 



