CELL-LINEAGE. 



29 



and gasteropods), while exactly conforming to the ancestraiybrw 

 of cleavage {i.e., that of the Turbellaria), differed toto ccelo from 

 it in morphological significance. When, some years ago, I first 

 called attention to this difficulty, I felt constrained to the ad- 

 mission that, in the face of such a contradiction, the study of 

 cell-lineage could only be regarded as of very restricted value 

 in morphological investigation ; indeed, in a lecture delivered 

 here four years ago on the inadequacy of the embryological 



Fig. 2.— Diagrams contrasting the value of the quartets in an anneUd or gasteropod (^) with 

 those of a polyclade according to Lang's original account {B). Lettering and shading as 

 in Fig. I. (The true proportions of the basal quadrants and the fourth quartet, which are 

 here misrepresented, are shown in Fig. 4. It is characteristic of the polyclades that the 

 fourth quartet-cells are greatly enlarged at the expense of the basal quadrants.) 



criterion of homology,^ I cited this very case as representing a 

 climax in the contradictions of comparative embryology. 



It is not rare in the history of science to find that fuller 

 knowledge may so change the point of view as to transform a 

 seeming difficulty into a pillar of support ; and it seems not 

 unlikely that such may be the case with the present one, though 

 some new difficulties have arisen which still await solution. 

 The new evidence relates, on the one hand, to the annelids and 

 mollusks, on the other hand to the polyclades ; and since on 

 both sides it tends to bridge a gap which once seemed hope- 

 lessly wide, I shall consider it in some detail. In approaching 



1 " The Embryological Criterion of Homology." Wood's Holl Biological 

 Lectures, 1894, p. 113. 



