EMBRYOLOGY OF THALASSEMA MELLITA 235 



in Capitclla, which have been described by Eisig ('98) as wander- 

 ing through the entodermal mass and finally emerging as func- 

 tional mesenchyme-cells. Not alone in behavior, but also in 

 appearance these cells are strikingly similar in both. Every 

 step in their degeneration in TJialasscma, however, has been fol- 

 lowed and there is no doubt of their complete absorption. 

 Another point of great interest, which should be taken into con- 

 sideration in an attempt at their interpretation, is the fact that 

 the cells, 1^1.1.2.2.1 ^^''d 1^1.1.2.2.1, which are typically rudimentary, 

 in the radial variety (about 9 per cent, of the embryos) are very 

 large and apparently entirely capable of function. Again the cell 

 I '^1.1.2.1.2, which is as a rule small but functional, may sometimes 

 be present as a rudiment (cf. PL I, Figs. 5, 6 and 7). In the first 

 case, evidently, cells which were once functional have become 

 functionless and in the second a cell manifests a tendency to 

 lose its function. From every point of view, accordingly, the 

 most reasonable explanation of the occurrence and the fate of 

 these rudimentary cells seems to be that they are vestigial. 



Child ('00) has expressed himself as rather sceptical of the 

 existence of true rudimentary cells. This is natural, as it would be 

 very difficult to account for their presence according to his theory 

 of the significance of spiral cleavage. " If they are formed," he 

 says, "it seems to me that the reason for their formation must 

 be other than the absence of cytoplasmic material." This is 

 doubtless true, but may we not go further and say, that pos- 

 sibly they are formed because of the presence of superfluous 

 cytoplasmic (or nuclear) material. This possibility was indi- 

 cated by Wilson ('98) several years ago in a paper in which 

 he has called attention to the fact that we may not only have 

 *' persistence in cleavage of vestigial processes in the formation of 

 the germ layers " (larval mesoblast of annelids and molluscs), but 

 also ''the persistence in cleavage of vestigial cells" (the rudi- 

 mentary enteroblasts of Aricia). He further says : " It would 

 be difficult to explain ancestral reminiscence in cell lineage by 

 any theory which does not recognize in cell outlines the definite 

 boundaries of differentiation areas in the developing embryo." 

 Lillie in a recent paper agrees with Wilson that these rudimen- 



