CELL-LINEAGE. 35 



blast of the annelid or mollusk, as a whole, corresponds with 

 that of the polyclade — in which case we must assume that in 

 the course of the phylogeny the posterior cell of the fourth 

 quartet has gradually taken upon itself more or less completely 

 the mesoblast formation formerly occurring in the second or 

 third quartet ; or the mesoblast of the polyclade has dwindled 

 away, perhaps has even disappeared, in the higher forms, where 

 it is represented only by the ectomesoblast, its place having 

 been taken, through a process of substitution, by the mesoblast- 

 bands derived from the fourth quartet. To vary the statement 

 we must assume that a substitution has taken place either in the 

 cell-mechanism by which the mesoblast is formed or in the meso- 

 blast itself, and upon our choice between these alternatives de- 

 pends the entire point of view from which we regard cell-lineage. 



Now, it must be admitted, forthwith, that we have not at 

 command sufficient data to give any certain answer to this 

 question, and we should be careful not to draw premature 

 conclusions in a matter which involves further consequences 

 of such importance. But there are a number of well-ascertained 

 facts drawn from widely diverse sources that point towards the 

 second of the above alternatives ; i.e., the view that the mesoblast- 

 bands of the annelid or gasteropod are not as such represented at 

 all in the polyclade, but, phyletically considered, are neomorphs 

 which have more or less completely replaced the ancestral meso- 

 blast. This evidence may be arranged in three lines : — 



I. As a result of exact and thorough studies upon the his- 

 tology and larval development of the annelids, Eduard Meyer 

 was several years ago led to the conclusion that the mesoblast- 

 bands, both in origin and in fate, differed widely from the scat- 

 tered larval mesenchyme-cells, though the lineage of the latter 

 was then unknown. Developing this idea, Meyer was led to 

 the remarkable conclusion that the mesoblast-bands of the 

 higher types represent the paired gonads of the ancestral form 

 — a view nearly related with the earlier one of Hatschek, that 

 the primary mesoblasts were originally eggs, which, in the 

 course of the phylogeny, became in part transformed into peri- 

 toneal and other somatic cells, and in part remained as germ-cells. 

 Thus the original mesoblast — ivhich Meyer definitely compared 



