. Annals of the New York Academy of Sciences. Science. Fig. 5. Diagram showing the typical arrangement of the micromere-quartets in polyclades, annelids and mollusks (their secondary divisions being omitted). A., from the upper pole. B, diagram of the typical history of the posterior quad- rant of an annelid or gasteropod embryo ; ectoblast is derived from the vmshaded cells (i, 2, 3), the mesoblast-bands from the dotted cell (4), ectoblast from the lined cells (5, Z>). the upper pole (diagram, Fig. 5). These quartets are dis- placed according to a definite law, the first being rotated, as
. Annals of the New York Academy of Sciences. Science. Fig. 5. Diagram showing the typical arrangement of the micromere-quartets in polyclades, annelids and mollusks (their secondary divisions being omitted). A., from the upper pole. B, diagram of the typical history of the posterior quad- rant of an annelid or gasteropod embryo ; ectoblast is derived from the vmshaded cells (i, 2, 3), the mesoblast-bands from the dotted cell (4), ectoblast from the lined cells (5, Z>). the upper pole (diagram, Fig. 5). These quartets are dis- placed according to a definite law, the first being rotated, as it were, towards the right (clockwise), the second towards the left (anti-clockwise), the third to the right, and so on in regular alternation.^ The secondary divisions of these micromeres also 1 There are some well-determined exceptions to this mode of cleavage, and at least one of these—the case oi Polychccnis, as described by Gardiner, 1895—is apparently irreducible to it. 2 The reversal of the direction of displacement in the sinistral gasteropods, dis- covered by Crampton, is an exception which emphasizes 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 New York Academy of Sciences. New York, New York Academy of Sciences
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