. Text-book of embryology. Embryology. 588 INVERTEBEATA chap. meres. Of these four, two, which later are shown to be anterior, are rather smaller than the others, so that even at this early period the egg is bilaterally segmented. These four blastomeres in turn divide into an upper and a lower tier, and so the 8-cell stage is attained; but the four upper cells, termed by Cerfontaine micromeres, are rather smaller than the four lower cells termed macromeres. There are therefore two larger and two smaller micromeres, and two larger and two smaller macromeres. In attaining the 16-cell stage each
. Text-book of embryology. Embryology. 588 INVERTEBEATA chap. meres. Of these four, two, which later are shown to be anterior, are rather smaller than the others, so that even at this early period the egg is bilaterally segmented. These four blastomeres in turn divide into an upper and a lower tier, and so the 8-cell stage is attained; but the four upper cells, termed by Cerfontaine micromeres, are rather smaller than the four lower cells termed macromeres. There are therefore two larger and two smaller micromeres, and two larger and two smaller macromeres. In attaining the 16-cell stage each cell divides into right and left daughters by radial planes, so that two tiers of eight cells should be formed; but four macromeres move downward and four micromeres upwards, so that four tiers of four cells each are formed. Each cell then divides into upper and lower halves, and in the ® ®. '^'^Plh'^^'^^^^^^>Cy^'^^^^^''^^'^P. 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 MacBride, E. W. (Ernest William), 1866-1940; Kerr, John Graham, 1869-; Heape, Walter, 1855-1929. London : Macmillan
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