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acceptance. We shall refer to this theory again in due time (cf. 
chapter III), 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 
wedge” 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 ís 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. 
Ff 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 
