DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 35 



do not finally become capable of independent motor reaction 

 and so separate and develop into complete individuals, as do 

 the zooids of Planaha, because of the greater effectiveness and 

 more rapid extension, with advancing development in the 

 annelid of nervous control from the apical region. Because 

 of this the segmental ganglia remain to a greater or less degree 

 subordinate constituent members of a more or less integrated* 

 whole. 



It is evident from the preceding discussion that the theory of 

 the axial gradients serves as a basis for the interpretation and 

 synthesis of a series of facts which otherwise remain merely 

 isolated data of observation and experiment. From this point 

 of view, the axial relations in the bilateral invertebrates, the 

 regional metabolic relations in the annelid egg, embryo and 

 larva, the origin of the segmental region from a certain region 

 of the egg, the formation of a posterior growing region with a wide 

 range of developmental potentialities and capacity for long 

 continued growth, and, lasth% the successive formation of seg- 

 ments, all appear to be connected and to have a common basis 

 in the axial gradients. Moreover, viewed in this light, annelid 

 development appears as the physiological activity of an organism 

 rather than as a mosaic of independent parts, a mysterious 

 predetermined harmony. The apparent mosaic condition is 

 secondary, not primary, concerns only certain regions of the 

 egg and moreover, is not a mosaic in the strict sense. In short, 

 the interpretation and synthesis of facts on this basis makes it 

 possible to discern a filndamental similarity between the phys- 

 iology of annelid development and that of other forms. 



Changes in the antero-posterior gradient after metamorphosis 



In the three-segmented larva the third or most posterior 

 segment is the most susceptible, the second less and the first 

 least susceptible. In Arenicola it has been found that, after 

 metamorphosis, in the worm of six segments the susceptibility 

 of the whole segmental region is higher than .in the three-seg- 

 mented stage, but the susceptibility decreases from the head 

 through the first bod3-segment to the third or fourth, while 



