. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 96 BIOLOGICAL LECTURES. annelids and molluscs, this "polar furrow" retains its original direction until a late stage of the development, and is of great service in orientation. At the next division each cell divides equatorially, so that there results an eight-cell stage, having four cells above and four below, the four upper being slightly smaller than the lower. (See Fig. 3 of an eight-cell stage from below.) Note especially the large size of D and the difference in size between the u
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 96 BIOLOGICAL LECTURES. annelids and molluscs, this "polar furrow" retains its original direction until a late stage of the development, and is of great service in orientation. At the next division each cell divides equatorially, so that there results an eight-cell stage, having four cells above and four below, the four upper being slightly smaller than the lower. (See Fig. 3 of an eight-cell stage from below.) Note especially the large size of D and the difference in size between the upper and lower quartettes. Because of this difference in size, we may speak of the upper quartette as micromeres, and. Fig. 3. — Amphiirite, eight cells from below. Fig. 4. — Fodarke, eight cells from the side. the lower quartette as macromeres, the distinction between the two quartettes being much less noticeable here than in some other annelids. This process is repeated during the formation of two more groups of micromeres, which form the body ectoderm, and a fourth group, one member of which forms the mesoderm, while the other three unite with the remaining macromeres to form the entoderm. Meanwhile cells earlier formed have divided, so that the separation of the mesoblast cell coincides with the completion of the ideal sixty-four-cell stage, a stage which, owing to delay in the division of some cells and a hastening of the division of others, may never actually be attained. A marked feature of this formation of micromeres is the division of each group in an alternating spiral direction. The eight-cell stage (Fig. 3) arises from the four-cell by a division. 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. ). Boston, Ginn & Co
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