. Text-book of embryology. Embryology. B la" lb". -2b Fig. 93. —Two stages in the segmentation of the egg of Cerebratulus lacteus viewed from the side. (After E. Wilson.) p, polar bodies. Aj 16-cell stage. B, 28-cell stage passing into 32-cell stage. another that it is quite impossible to distinguish an A, a B, a C, and a D segment, consequently the naming of the quadrants in the segmenting egg is an arbitrary matter. From these four macromeres quartettes of micromeres are budded off—the first quartette dexio- tropically, the second laeotropically, until no less than six quartettes h
. Text-book of embryology. Embryology. B la" lb". -2b Fig. 93. —Two stages in the segmentation of the egg of Cerebratulus lacteus viewed from the side. (After E. Wilson.) p, polar bodies. Aj 16-cell stage. B, 28-cell stage passing into 32-cell stage. another that it is quite impossible to distinguish an A, a B, a C, and a D segment, consequently the naming of the quadrants in the segmenting egg is an arbitrary matter. From these four macromeres quartettes of micromeres are budded off—the first quartette dexio- tropically, the second laeotropically, until no less than six quartettes have been formed (Fig. 93). The exact fate of these quartettes is not described in detail by Wilson. He remarks, however, that the endoderm extends to the equator of the spherical embryo. From this we infer that, as usual, the upper or ectodermic half of the segmented egg is constituted by the three first quartettes of micromeres; and that the fourth, fifth, and sixth quartettes, together with the residual macromeres, con- stitute the endoderm. The residual macromeres are similar in size and appearance to the last quartette of micromeres. In this way a hollow sphere of cells, the blastula, is formed which is uniformly ciliated. Then one side of it becomes flattened and the. 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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