72 THE ANCESTRY OF VERTEBRATES 



acceptance. We shall refer to this theory again in due time (cf. 

 chapter HI), I can only say here that to me also it seems 

 untenable, the roof of the archenteron being in my opinion 

 wholly entodermal. Further, the three authors mentioned think 

 they can distinguish very sharply in the roof of the archen- 

 teron the invaginated ectodermal cells (the "protochordal 

 v^edge" of HUBRECHT)from the entodermal cells (the "proto- 

 chordal plate", H.), and to be able thus to trace exactly the 

 limit of the two. It is subject, however, to serious doubt, 

 whether the histological character of cells, in this case the 

 yolk-richness, thus permits us to trace their origin from the 

 outer or the inner germinal layer with equal certainty as e.g. 

 cell-lineage investigations allow us to do in Annelidan 

 development. But assuming the above authors were right as 

 far as concerns the facts, then, as stated above, we still would 

 reach the conclusion, that in Vertebrates the headmesoderm 

 ("Urmesoderm" of De Lange) would be of entodermal 

 and the coelomesoderm of ectodermal origin, while in 

 Annelids the reverse is true, the primary or larval mesoderm 

 being of ectodermal, the secondary or coelomesoderm of 

 entodermal origin. 



If indeed in the head of Vertebrates a headmesoderm, 

 comparable to the primary or larval mesoblast of Annelids 

 and Molluscs, were present, it should be of ectodermal 

 origin, whereas the coelomic mesoderm, in accordance 

 with what is found in Annelids, is wholly of entodermal 

 origin. Now it has been held by several authors, who 

 follow in this Miss Platt (1897), that ectodermal mesen- 

 chyme is proliferated from the lateral placodes which 

 contribute to the formation of the head-ganglia. Yet it 

 seems to me hardly probable that, if these statements are 

 right (others have opposed to them, cf. BUCHS, 1902, p. 605), 

 we have to do here with primary head-mesoderm compar- 

 able to that of Annelids and to the mesenchyme of their, 

 non-segmented ancestors, the stage of development in which 

 it appears being much too late. It seems to me improbable 

 that traces of the primary ectomesoderm should be still 

 found in Vertebrates. In Amphioxus we stated already 

 that in the prostomium (fig. 5) no mesoderm is found, 

 that all the mesoderm of the embryo is coelomesoblast and is 

 of entodermal origin and belongs to the trunk. Only later, 

 according to Hatschek (1882), a forward prolongation of 

 the first somite provides the snout with mesoblast, in the 



