EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 4] 
brings about a state of things in which it very soon becomes 
utterly impossible to say to which of the three centres a given 
cell or group of cells owes its origin. An intimate fusion, 
though withdrawing this question of cell-lineage out of the 
field of our powers of discrimination, does not, however, dimi- 
nish the significance of the existence of such a cell-lineage,! 
and we will in future researches have to keep our attention 
directed to that point, even though we must recognise at the 
present moment that much of the confusion and of the erro- 
neous notions that maintain such a hazy atmosphere round 
these important early phases of vertebrate development, is 
due to precocious generalisations on this head. It seems to 
me that the wish to uphold the reality of a third germinal 
layer, together with the ardent desire of not having to 
ascribe a multiple origin to it, is responsible for much theo- 
retical dogmatism that will henceforth prove valueless. 
The consequence of what we have here described for 
Tarsius is that the centres of proliferation which give rise to 
the protochordal wedge and to the ventral mesoblast are 
originally independent of each other. We shall by-and-bye 
see that there is all reason to believe that the same holds 
good for all other Mammalia, aye, for all other Vertebrates. 
The principal difference between my own and the current 
views consists in the distinction which I wish to make 
between what was considered as the front portion of the 
primitive streak (Hensen’s knob, of which even the anterior 
prolongation was called in full: ‘* Kopffortsatz des Primitivi- 
treifens”’) from the primitive streak material itself. This 
distinction, which is very soon effaced and could never be 
demonstrated in later stages, is, however, quite evident in the 
very early ones. And we will have to analyse, as acutely as 
we can, the differences this will call forth in our interpretation 
of the development of different tissues and organs concerned. 
The ventral mesoblast at its very earliest appearance (also 
in Tarsius) may be said—as it springs from the hinder end 
of the ectodermal shield—to be more or less crescent- or fan- 
1 Vide E. B. Wilson (’92, °97), as against Driesch and others. 
