DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 31 



along its course is greater in degree and less readily altered or 

 obliterated than in the latter. Nevertheless, the differences in 

 metabolic rate in different regions still play a role in further 

 development, even in the annelid, though a less fundamental 

 role, except in the posterior region, than in the sea urchin. 



The origin of the segmental region in relation to the axial gradients 



It is perhaps a significant fact that the cells which consti- 

 tute the posterior growing region arise from the dorso-posterior 

 region of the egg, the ectoblast being the most anterior, the meso- 

 blast further posterior and the entoblasts most posterior of all. 

 In the flatworms, and probably also in other bilaterally sym- 

 metrical invertebrates, the region of highest susceptibility and 

 metabolic rate in the axes of bilaterality is primarily the ventral 

 region, and susceptibility and metabolic rate decrease laterally 

 and dorsally (Child, '13; '15 b, pp. 60, 67 to 69). This is true, 

 at least in the earlier stages of development, though changes 

 may occur later. In such forms, then, the dorso-posterior 

 regions are the regions of lowest metabolic rate in the body. In 

 the annelid egg the cells which take almost no part in the forma- 

 tion of the rapidly differentiating, specialized strictly larval 

 structures, arise from the posterior region of the egg. As re- 

 gards the relative susceptibility of this region, it is certainly 

 true that the somatoblast from which the ectoderm of the growing 

 region arises is, in many cases, less susceptible when first formed 

 than other parts of the ectoderm; the mesoblast is still less 

 susceptible and the entoblasts are least susceptible of all. If the 

 symmetry gradient as well as the polar gradient is present in the 

 egg from the beginning of development, then these cells represent, 

 in general, the region of lowest metabolic activity in the egg 

 cytoplasm, and the ectodermal somatoblast arises from the 

 most active part of this region, the entoblasts from the least 

 active and the mesoblast from a region intermediate between 

 the two. This region is least involved in the larval specialization 

 and is therefore capable of undergoing a greater degree of re- 

 juvenescence and so becomes more embryonic. It possesses 

 in all cases a wide range of developmental potentialities and is 



