Abstract
1. Isolation experiments on the egg of Sabellaria vulgaris demonstrate that the formation of the apical tuft in partial larvæ is dependent upon the presence of the first polar lobe and the C cell; that the post-trochal region develops only when the three polar lobes and the 1D cell are present; and that apical cilia form only if the A or B cell is included. 2. Form changes in isolated first and second polar lobes are described. The early changes are synchronous with the cleavages of the ovum, except that all events in the isolated lobe are delayed. 3. The results of the following transplantation experiments are reported: (a) Transplantation of polar lobes. (b) Transplantation of blastomeres to the whole egg. (c) Transplantation of blastomeres to E-PL1. (d) Fusions of half- and quarter-blastomeres. (e) Fusions of two eggs. In all combinations, complete self-differentiation of individual blastomeres occurs. Apical tufts develop only when C cells are present, post-trochal bristles are dependent upon the presence of D cells, and apical cilia form only when either the A or B cell is included. 4. The results of this investigation are compared with those from experiments on other mosaic eggs.
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