. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 20 the antero-posterior direction, and the anal arms begin to develop, these arms and the posterior end both appear as secondary regions of high susceptibility (Fig. 20) though the susceptibility of the anal arms is in general somewhat less than that of the oral lobe and that of the posterior end somewhat less than that of the anal arms. On the oral lobe and over the body death progresses in the basal and posterior direction and in the anal arms from tip to base of the arms. In the fully developed pluteus the susceptibility
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 20 the antero-posterior direction, and the anal arms begin to develop, these arms and the posterior end both appear as secondary regions of high susceptibility (Fig. 20) though the susceptibility of the anal arms is in general somewhat less than that of the oral lobe and that of the posterior end somewhat less than that of the anal arms. On the oral lobe and over the body death progresses in the basal and posterior direction and in the anal arms from tip to base of the arms. In the fully developed pluteus the susceptibility gradients are less marked. The ectoderm of the oral lobe and anal arms is still somewhat more susceptible than that of other regions but the differences are less conspicuous. In all these later stages, how- ever, the entoderm remains much less susceptible than the ecto- derm and apparently the mesenchyme is least susceptible of all parts. It is probable that the gradual fading out of the metabolic gradients in the pluteus is a physiological change which precedes and makes possible the development of the axial gradients of the mature sea urchin which have been previously inhibited by the existing axial relations. If this suggestion is correct these changes are the factors which determine, or rather permit metamorphosis. The development of the arms and the posterior elongation has been shown to be dependent on the development of the skeleton. That being the case the appearance of high suscepti- bility in the anal arms and the posterior end is not self-determined in these parts but results from skeletal Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Marine Biological Laboratory (Woods Hole, Mass. ); Marine Biological Laboratory (Woods Hole, Mass. ). Annual report 1907/08-1952; Lillie, Frank Rattray, 1
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
