CELL-LINEAGE. 



39 



blastic part.^ Such a series creates a strong probability that we 

 have before us a vanishing series Hke those so well known in 

 adult organs, such as the limbs, the tail, or the teeth. Further, 

 just as the lateral toes of the horse seem to have wholly vanished, 

 even from the ontogeny, so the vestigial entoblasts would seem to 

 have disappeared in some annelids and mollusks, leaving the 

 posterior cell of the fourth quartet purely mesoblastic. 



These considerations invest with a special interest the cor- 

 responding cell in the Tiirbellaria (i.e., the posterior member 

 of the fourth quartet, ^d) ; and this interest is heightened by 

 Lang's discovery that in Discoccelis this cell divides earlier 

 than the other cells of the quartet, and into equal halves which 

 lie symmetrically at the posterior end of the embryo. These 

 two cells thus correspond exactly in origin and position with 

 the paired mesentoblasts of the annelids and gasteropods, and 

 the facts naturally led to the suggestion, made by Mead, that 

 they would perhaps be found to give rise to paired mesoblast- 

 bands, as in the higher types. In Leptoplana (Fig. 4, D, E, F) 

 a similar division occurs, but as far as their fate is concerned 

 my own observations do not sustain Mead's suggestion, on the 

 one hand giving no evidence that these cells give rise to any- 

 thing other than the posterior cells of the archenteron, on the 

 other showing that they are often unequal or asymmetrically 

 placed (Fig. 4, D, E) and only rarely conform to Lang's scheme 

 (Fig. 4, F). If, therefore, the polyclades represent the ances- 

 tral type in this respect, we must conclude that the entomeso- 

 blast was a later development. The remarkable fact is that, if 

 such has been the case, the new mesoblast-formation has been 

 fitted, as it were, upon an old form of cleavage occurring regu- 

 larly in Discoccelis and occasionally in Leptoplana. The two 

 symmetrical posterior entoblast-cells of the polyclade might 

 thus be conceived as the prototypes of the primary mesoblasts 

 or mesentoblasts of the higher forms, which in the course of 

 the phylogeny undertook the formation of mesoblastic as well 

 as of entoblastic elements. ^ The old building pattern was still 



1 1 am here placing my own interpretation on Mead's and Lillie's observations. 



2 Lang has pointed out a motive for this form of cleavage in the polyclade, 

 correlating the early and symmetrical division of d"" with the posterior bifurcation 

 of the gut. 



