Journal of morphology . gs, and to verify their accuracy in respect to some of themost important stages by the study of the original preparations. This is by far themost accurate and complete study of the molluscan cleavage that has thus far beenmade. The resemblance to the Nereis cleavage is so close and extends over so longa period as to be most extraordinary. Since, however, the figures may not be pub-lished for some time, I am compelled for the present to take Blochmanns Neritinaas my principal basis of comparison. I may add that, in the course of the pastsummer, Conklin has ascertained th
Journal of morphology . gs, and to verify their accuracy in respect to some of themost important stages by the study of the original preparations. This is by far themost accurate and complete study of the molluscan cleavage that has thus far beenmade. The resemblance to the Nereis cleavage is so close and extends over so longa period as to be most extraordinary. Since, however, the figures may not be pub-lished for some time, I am compelled for the present to take Blochmanns Neritinaas my principal basis of comparison. I may add that, in the course of the pastsummer, Conklin has ascertained that the first cleavage-plane in Crepidnla is notlongitudinal, as he at first described it, but transverse, precisely as in Nereis. Thepreparations he has shown me seem to leave no room for doubt on this point. 440 WILSON. [Vol. VI. handed, and the third right-handed again. The first two setsof cells thus produced alternate with each other, and thus forma second or inner belt of eight cells {C, D. E), interposed between 1. Diagram VII. A. Diagram of the polyclade embryo, showing the relations of the micromeres tothe macromeres. Mesoblast shaded. The Roman numerals designate the cleavage-plans. B. Corresponding diagram of the Nereis embryo. C. Twenty-eight-celled stage of polyclade. D. MoUuscan embryo after the differentiation of the germ-layers. E. Corresponding diagram of Nereis. No. 3-] THE CELL-LINEAGE OF NEREIS. 441 the central (primary) micromeres and the outer belt. Finally,the second set of micromeres divide meridionally, and in Nereisand Crepidiila this can be distinctly recognized as a left-handedspiral. Thus arise four pairs of cells, with which alternate thefour micromeres of the third set; and the outer belt now con-sists of twelve cells, which form the lips of the blastopore(C, D,E). All three agree, furthermore, in the meridional division of thefirst-formed progeny of the primary micromeres {p)--^, b^-^, c^-^,d^-^), so that the inner belt is likewise composed of twe
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