PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 139 



general ectoderm about them. As the two coelom sacs develop from the 

 thin-walled apical archenteron, they seem to reduce slightly more rapidly 

 than the adjoining entoderm; but this is somewhat uncertain because the 

 whole apical archenteron at these stages reduces very rapidly, often show- 

 ing reduction while staining in open solution, probably because oxygen 

 content in that part of the enteric cavity is below the critical level. Here, 

 as in the echinoids, the gradients associated with development of organs 

 become less distinct or disappear with termination of larval development. 

 Apical and ventral regions usually still reduce somewhat more rapidly 

 than the dorsal ectoderm. Some changes in entodermal reduction occur, 

 probably associated with regional differentiation. Reduction during met- 

 amorphosis has not been observed. 



It seems evident that these reduction gradients and their changes indi- 

 cate some of the quantitative features of echinoderm developmental pat- 

 tern. The changes in the gradient systems in the course of development 

 and their close association with development of morphological form are 

 of particular interest. However, they give no direct evidence concerning 

 absence or presence of regional specificities or qualities, though it seems 

 highly probable that there is a relation of some sort between them and 

 regional differentiation. In general, the gradients appear to precede any- 

 thing clearly distinguishable as differentiation. 



The earlier observations on susceptibility in A rhacia and A sterias gas- 

 trulae agree with these more recent data on dye reduction, so far as ecto- 

 derm is concerned. Susceptibility was found to decrease from the apical 

 region of the A sterias entoderm of middle and later gastrula stages; and 

 later, unpublished observations showed high susceptibility about the 

 A sterias blastopore. Developing oral lobe and anal arms of Arbacia 

 showed basipetal susceptibility gradients. 



According to a recent study of dye reduction in development of two 

 other species of sea urchins, dye reduction progresses from the apical pole 

 basipetally throughout larval development, with no indication of a sec- 

 ondary acropetal gradient in late blastula and gastrula. It is also described 

 as progressing from the outer cell surfaces inward, instead of from within, 

 outward, as observed by Child (Ranzi e Falkenheim, 1937). These au- 

 thors find no evidence of an indophenol blue gradient, as might have been 

 expected, for concentration of the reagent used was so high that it cer- 

 tainly killed at once, and dead embryos and larvae show no indophenol 

 gradients or only the merest traces of them. Only use of the reagents in 

 very high dilutions will show the gradients in living animals. Also, no 



