. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. CRYSTAL ORIENTATION IN SEA URCHIN 407 on the circumference of the spine. Each wedge of the spine shows the same ori- entation. Since each of the three angles formed by the intersection of the three cleavage faces were 120° it was judged that the observations were made parallel to the electron beam along the c-axis of the spine. One can accordingly conclude from the correspondence of the angles that the spine is a single crystal whose c- axis is normal to the cross-section, that is, parallel to the axis of elongation of the s
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. CRYSTAL ORIENTATION IN SEA URCHIN 407 on the circumference of the spine. Each wedge of the spine shows the same ori- entation. Since each of the three angles formed by the intersection of the three cleavage faces were 120° it was judged that the observations were made parallel to the electron beam along the c-axis of the spine. One can accordingly conclude from the correspondence of the angles that the spine is a single crystal whose c- axis is normal to the cross-section, that is, parallel to the axis of elongation of the spine. Further, since the angle between the a-axis and the ridge of calcite is 30° when viewed along the c-axis (Fig. 3), etch figures can also be used to determine the a-axes. From etch figures in Figures 2C, D, the three a-axes of Figure 2A were determined and are indicated by the superimposed dark lines of Figure 3. Because the etch figures were relatively small, it was necessary to make many measurements from a single micrograph for exact determinations of cleavage sur- faces. By decorating the etched surfaces, determinations of c- and a-axes were greatly facilitated. The rhombohedral decorated calcite crystals were of sufficient size and the coigns prominent to the extent that the stage of the electron microscope could be oriented so that the ridges of single crystals formed 120° angles with one another. Thus, one could assume that the crystal was being observed along its c- axis. The a-axes could then be calculated. Figures 2E, F are representative of the habits of the crystals as observed at all locations on the cross section of a single spine, including its base. This result supports the conclusion based on etch figures that the spine is a single crystal. Although calcite crystals could be grown on unetched surfaces of the spine, crystals of varying orientation frequently developed, presumably from small chips produced by cutting which served as nuclei. To test the re
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
