128 PATTERNS AND PROBLEMS OF DEVELOPMENT 



polychetes. In unidentified syllids individuals developing from buds 

 showed the U-shaped gradient after the posterior segment-forming region 

 was established. The two ends of N. virens are more susceptible to cyanide 

 than the middle.^'^ 



If it be granted that a U-shaped gradient involving metabolism is pres- 

 ent in the body walls of adults of a considerable number of annelid species 

 and that the posterior arm of the gradient is related to the progressive 

 formation and development of new segments, there is every reason to be- 

 lieve, as was noted years ago by Hyman (1916a), that the two arms of the 

 gradient are not identical in kind of metabolism. The anterior arm appar- 

 ently represents the final expression of the primary embryonic polar gradi- 

 ent in this fully differentiated region; the posterior arm represents differ- 

 ent stages of growth and differentiation of segments, the earhest stages 

 being anterior to the anal segment, with stages of development and physi- 

 ological age progressively more advanced from segment to segment ante- 

 riorly as long as segment formation and development continue. It would 

 be most surprising if there were not a metabolic gradient in this region 

 during the period of segment formation and development. But in those 

 species in which segment formation ceases, either at hatching or at some 

 later stage, and all segments sooner or later attain full development, this 

 posterior arm of the gradient may disappear completely, and only a sim- 

 ple gradient from anterior to posterior end may be present ; or, with special 

 differentiations and functions of particular regions other gradient modifi- 

 cations may appear. In Tubifex, for example, development of the clitellum 

 alters the gradient of the body wall in the region where it appears. Deter- 

 minations of oxygen uptake, CO, production, differential dye reduction, 

 and differential susceptibility show only certain quantitative factors and 

 give no information concerning other axial differences, which may be and 

 doubtless are, present in the adult. That the metabolism indicated by 

 oxygen consumed or CO2 produced is the same in character in anterior and 

 posterior regions of an annelid body does not follow from the data and is 

 not assumed; but that there are differences in basal rate, that the rate de- 



'i Parker, 1929. This author concluded that the higher susceptibility of anterior and pos- 

 terior regions resulted from entrance of cyanide through the mouth and anus. This is 

 certainly not the case in other forms: death and disintegration begin on the outer surface of 

 the body; the posterior arm of the susceptibility gradient of Tubifex and other microdrilous 

 oligochetes extends over more than half the body length; that cyanide or other agents should 

 be carried so far anteriorly in the intestine is extremely improbable. Moreover, differential 

 death of the body wall may occur in vital dyes without staining of the intestine. Also, differ- 

 ential susceptibility to lack of oxygen is the same as to cyanide. 



