CELL- LI XE A UE. 23 



may have preponderated as greath' over the mesobhistic as the 

 latter now preponderates over the entoblastic in Aricia ; and that 

 the beginning of the series may have been sucli a mode of develop- 

 ment as still occurs in the polyclade where the entire quartet 

 is entoblastic. Thus we are brought anew to the view which 

 has been advocated by a number of morphologists, prominent 

 among them Edouard Meyer/ that the mesoblast-bands (ento- 

 mesoblast) of the higher forms may have been of different origin 

 phylogenetically from the "larval mesenchyme"- 



More specifically I would suggest that in the ancestral type 

 the fourth quartet was strictly entoblastic ; that at a later period 

 in the phylogem-the trunk-mesoblast (mesoblast-bands of higher 

 types) took its origin from the posterior part of the archenteron, 

 perhaps in connection with the development of a new body-region 

 from the posterior part of the ancestral body ; and that as the 

 cleavage became progressively specialized (/". c, assumed more of 

 what Conklin has termed a '* determinate type ") the seat of this 

 mesoblast-formation became more and more definitely localized 

 in the posterior member of the fourth quartet. The symmet- 

 rical division of this cell in the polyclade might accordingly be 

 regarded as the prototype of that which occurs in the annelid 

 or mollusk, though the resulting cells have in the latter 

 forms acquired a different morphological significance. In other 

 words the old building-pattern, still persisting more or less 

 definitely in the polyclade, has been adapted to a new use'^ 

 precisely as in the evolution of adult structures. 



I would distinctly repeat that these suggestions are offered only 

 as a speculative working hypothesis ; yet, despite their hypothe- 

 tical character, it seems to me that they may give a new point of 

 attack upon some of the puzzling phylogenetic problems with 

 which the study of cell -lineage has to grapple. 



1 1890, p. 299. 



^t Cf. Conklin, p. 15 1. 



•^"Imagine that in any species anew organ is added, or rather, that a diffuse 

 series of stiiictures gains great importance and compactness in the course of evolu- 

 tion. Then this new structure viay be represented in ontogeny by a cell. But the 

 form of cleavage is already defined. -^ ^ * The manufacture of a new cell be- 

 ing an impossibility, an old cell must be modified to represent the new organ." 

 (Lillie, 1895, P- 37) 



