2 WILSON. 



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 sug- 

 gested that the pair of rudimentary cells in 'Aricia and Spio 

 were to be regarded as vestiges of an ancestral type of devel- 

 opment 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 em- 

 bryology. 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 " mechan- 

 ical " 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 f and they also raise some 

 interesting further questions regarding ancestral reminiscence in 

 cell-lineage. I am also able to contribute some new observa- 

 tions on the cell-lineage of a polyclade {Leptopland), 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 A'', viegalops, 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 

 Flanorbis (1897, p. loi). Lillie has found a pair of corresponding but shghtly 

 larger cells in Unio (1895, p. 27), while in Clymenella they are as large as the pri- 

 mary mesoblasts (Mead, 1897, p. 264). The corresponding cells in Umbrella 

 (Heymons), Crepidtda (Conklin), and Physa (Wierzejski) will be referred to be- 

 yond (see pp. 6, 11-12). 



