CELL-LIXEAGE. 25 



Recent advances in the study of cell-lineage have, it is true, 

 raised some new apparent difficulties in the attempt to establish 

 precise cell-homologies, even between nearly related forms^ 

 though I suspect that some of these will be found less serious 

 than they now appear. Against these difficulties, hovvev^er, 

 may fairly be placed an increasing body of affirmative evi- 

 dence,- and on this side may be ranged the observations re- 

 corded in the present paper. We should, moreover, remember 

 that just as the homologies of adult parts may be complete or 

 incomplete in various degrees (as Gegenbaur long since urged), 

 so cell-homologies may be more or less definite. Furthermore, 

 just as we cannot always find exact equivalents, in related forms, 

 of the several sub-divisions of homologous nerves or blood- 

 vessels or sense-organs, so we need not expect to find exact 

 homologues for all the individual cells throughout ontogeny. 

 The wonder is, indeed, that so many definite cell-homologies 

 have been established. I believe the facts now known demon- 

 strate the inadequacy of Hertwig's too simple conclusion that the 

 definite values of the blastomeres, and hence of the cell-homol- 

 ogies based upon them, are merely an incidental result of the 

 continuity of development,"^ and that they do not leave without 

 support the plea made five years ago in my paper on Nereis, for 

 the study of cell-lineage as a guide to relationship.'^ 



In the second place, these facts seem on the whole to em- 

 phasize the importance of cell-formation in development. The 

 inadequacy of the cell -theory as applied to development has 

 been very ably urged, especially by Whitman and by Adam 

 Sedgwick ; and their conclusions, fortified by the epoch-making- 

 discoveries of Roux, Driesch and others on the development of 

 isolated blastomeres, are of an importance that we are only be- 

 ginning fully to realize. But the time has not yet come for a 

 just estimate of the cell-theory in this aspect ; and it may well 

 be questioned whether in the reaction against the cell -mosaic 

 theory, as originated by Schwann, and developed with so much 



iCf. Mead, 1897, and Child, 1897. 



2Cf. Conklin, 1897. 



3 Cf. the very eft'ective criticism of ConkUn, 1897, p. 191. 



^1892, pp. 367, 455. 



