CONSIDERATIONS ON Cell-Lineage and Ancestral Reminiscence, BASED ON A RE-EXAMINATION OF SOME POINTS IN THE EARLY DEVEL OPMENT OF ANNELIDS AND POLYCLADES, BY EDMUND B. WILSON, ANNALS NEW YORK ACADEMY OF SCIENCES, XI., No. 1. LANCASTER, PA. THE NEW ERA PRINTING COMPANY. [ANNALS N. Y. Acad. Sci., XI., No. 1, pp. 1–27, March 30, 1898. ] CONSIDERATIONS ON CELL-LINEAGE AND ANCESTRAL REMINISCENCE, BASED ON A RE-EXAMINATION OF SOME POINTS IN THE EARLY DEVELOPMENT OF ANNELIDS AND POLYCLADES. EDMUND B. WILSON. (Read December 13, 1897.) FIVE years ago I observed in the embryos of two polychatous annelids, Aricia fœtida (Clap.) and Spio fulginosus (Clap.), that the two so-called "primary mesoblasts" bud forth a pair of extremely minute superficial cells near the posterior lip of the blastopore before giving rise to the mesoblast-bands.1 Scarcely larger than polar bodies, these cells lie at or near the surface at the posterior margin of the entoblast-plate, wedged in between the latter and the primary mesoblasts (Fig. 1, A, C, e; Fig. 2, A, e, e); and in this position they are carried into the interior during the ensuing invagination. I could not determine their fate, and found no evidence that they underwent growth or division, or that they took any part in the building of the embryo. In Nereis, however, I found that this pair of rudimentary cells was represented by a group of not less than six or eight somewhat larger cells (Fig. 1, B, D; Fig. 2, B), formed in exactly the same way and in the same position, and further that these 21892, p. 411. 11892, p. 458. cells were functional in development, giving rise to a definite part of the body, though, as will appear beyond, I fell into error regarding their precise fate.' These facts strongly suggested that the pair of rudimentary cells in Aricia and Spio were to be regarded as vestiges of an ancestral type of development in which they were represented by a group of larger functional cells, such as are still found in the embryo of Nereis. Such a conclusion, if it could be established, would possess an importance for the general problems of cell-lineage even greater than its interest for the more special problems of annelid embryology. For if vestigial structures may appear in ontogeny in the form of single cells, the fact would not only afford a striking illustration of the inadequacy of all so-called "mechanical" explanations of cleavage-forms, but would supply a very important datum for the estimation of the cell-theory as applied to development. The results of a re-examination of the history of these small cells in Nereis, taken in connection with other recent studies in cell-lineage, lend strong support to the conclusion indicated above, enabling us, as I believe, to give a definite interpretation to the vestigial cells of Aricia, Spio and other forms in which they have recently been observed; and they also raise some interesting further questions regarding ancestral reminiscence in cell-lineage. I am also able to contribute some new observations on the cell-lineage of a polyclade (Leptoplana), which bear directly on these questions and considerably extend their range. 1 Von Wistinghausen (1891 ) had previously observed in Nereis Dumerilii, a group of small cells derived from the "second somatoblast," which probably correspond with those I have described in N. limbata and N. megalops, though their exact origin was not followed. Wistinghausen believed that they gave rise to a part of the ectoblast-a result wholly different from both my earlier account and the present one. 2 Minute cells exactly corresponding in origin and number to those of Aricia have been found by Mead in Amphitrite (1894, p. 467; 1897, p. 247) and by Holmes in Planorbis (1897, p. 101). Lillie has found a pair of corresponding but slightly larger cells in Unio (1895, p. 27), while in Clymenella they are as large as the primary mesoblasts (Mead, 1897, p. 264). The corresponding cells in Umbrella (Heymons), Crepidula (Conklin), and Physa (Wierzejski) will be referred to beyond (see pp. 6, 11-12). |