CELL-LINEAGE. 13 



The bearing of this conclusion on the possible relation be- 

 tween the teloblastic and enterocoelic modes of mesoblast-for- 

 mation is obvious. This question will, however, appear in a 

 clearer light after a consideration of the polyclade cell-lineage 

 in relation to the foregoing results. 



II. 



The MlCROMERE-OUARTETS IN AxNELIDS, MoLLUSKS AND 



POLYC LADES. 



The marvelously close resemblance in cell-lineage between 

 the annelids, gasteropods and lamellibranchs which recent re- 

 search, more especially within the last five years, has brought 

 to light, leaves no doubt not only that the general forms of 

 cleavage in these groups are reducible to a common type, but 

 also that a considerable number of more or less definite cell- 

 homologies can be established between them, even in the early 

 cleavage-stages. The attempt to extend the comparison beyond 

 the limits of these groups has, however, thus far encountered a 

 very serious stumbling-block in the cell-lineage of the poly- 

 clades. If we accept Lang's view, which is supported by a 

 large amount of evidence, that the platodes are not very far 

 removed from the ancestral prototype of annelids and moUusks, 

 we should expect to find in the polyclade a mode of cleavage 

 to which that of the higher forms can in its main features be 

 reduced. In point of fact, however, this seems to be the case 

 only in the /<?;';;/ of cleavage and not, so to speak, in its substance ; 

 for, although the general type of cleavage and the arrangement 

 of the blastomeres in the polyclade shows an extraordinary re- 

 semblance to that of the annelid or gasteropod, the cells seem 

 not to have the same morphological value. I have elsewhere 

 sufficiently indicated the nature of this difficulty,^ which has 

 also been remarked by a number of other writers ; but for 

 the sake of clearness I will again direct attention to its leading 

 features. 



"^Nereis, p. 441 ; The Cell, pp. 314, 315. 



