[Annals N. Y. Acad. Sci., XL, No. i, pp. 1-27, March 30, 1898.] 



CONSIDERATIONS ON CELL-LINEAGE AND 

 ANCESTRAL REMINISCENCE, 



BASED ON 



A Re-Examination of Some Points in the Early Devel- 

 opment OF Annelids and Polyclades. 



Edmund B. Wilson. 



(Read December 13, 1897.) 



Five years ago I observed in the embryos of two polychae- 

 tous annelids, Aricia fcetida (Clap.) and Spio fulginosiis (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.^ 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. i, A, C, e ; Fig. 2, 

 A, e, c) ; 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 underw^ent growth or di- 

 vision, 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 some- 

 what larger cells (Fig. i, B, D ; Fig. 2, B), formed in exactly 

 the same way and in the same position,^ and further that these 



11892, p. 458. 21892, p. 411. 



