28 C. M. CHILD 



normal development cannot be doubted. In the metamorphosis 

 of the polychete a decrease in size of the head-region is a very 

 characteristic feature. It appears in Chaetopterus (figs. 12 

 to 14), Nereis (figs. 24 to 27) and Arenicola (figs. 48 to 50), 

 and in some annelids, for example Polygordius, it is much more 

 marked than in these three. In the light of what the suscepti- 

 bility method has shown us concerning the changes in metabolic 

 relations in the different regions of the body during develop- 

 ment, there is every reason to believe that the reduction of the 

 head-region in metamorphosis is an indirect result of increase 

 in activity of the posterior growing region, until it is the most 

 active region of the body. Under these conditions, particularly 

 in the later stages when the nutritive supply of the egg is ap- 

 proaching exhaustion, the posterior growing region and the more 

 posterior segments apparently grow to a greater or less extent 

 at the expense of the head-region and the more anterior segments. 

 Probably the cephalic ganglia, which undoubtedly represent a 

 region of higher metabolic rate than the rest of the head ecto- 

 derm, also grow to some extent at the expense of other parts of 

 the head. 



If we alter the normal relations by inhibiting both head and 

 posterior growing region more than the anterior segmental region, 

 as when the inhibiting action begins at the beginning of de- 

 velopment, the latter may become not merely relatively, but in 

 time absolutely larger than in the normal animal, as in the case 

 in many of the figures above. If, on the other hand, we inhibit 

 the posterior growing region to a greater extent than the head 

 and anterior region, as in the cases where inhibition begins with 

 the early trochophore stage, these regions may become absolutely 

 larger than in the normal larva and their reduction may be de- 

 layed or may be less than normal as long as the differential 

 inhibition persists. This condition also appears in the figures. 

 If this interpretation be correct, it is evident that, at least from 

 the metabolic point of view, the larval development of the poly- 

 chete is not a mosaic of independent parts as Wilson and others 

 have maintained. 



