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tion of the germinal layers in Vertebrates be not exhaustive, 
lthink it will sufficiently illustrate the reigning confusion 
and uncertainty on these subjects. The cause of all this 
controversy is the absence of true insight as a consequence 
of the lack of a phylogenetic guiding thread connecting the 
Vertebrates with the lower groups of animals. Observations 
and facts alone can not help us here. My theory leads to 
the following conception which, I believe, also proves to 
be best ín harmony with the facts. 
In Invertebrates the blastopore, after having contracted 
to a narrow opening, passes into the cardiac pore as a con- 
sequence of the formation of the stomodaeum. In Vertebrates 
the blastopore, after having contracted to a narrow opening, 
passes into the neurenteric pore as a consequence of the 
formation of the medullary tube. In Invertebrates we call 
gastrula the stage in which the blastopore has contracted 
to a narrow opening but before the stcmodaeum has formed. 
Thus in Vertebrates we must call gastrula the corresponding 
stage, being e.g. that of fig. 12 or Plate II, fig. 1. That which 
lies at the surface here is the primary ectoderm, that which 
lies in the interior is the primary endoderm, just as in 
Invertebrates. Thus the roof of the archenteric cavity 
consists of (primary) endoderm’ cells just as does the floor, 
as has been argued already by many investigators who have 
emphasized the fact that, e.g. in the frogs egg, the cytological 
character also of the cells of the roof renders it quite evident 
that they belong to the endoderm, not to the ectoderm. 
Eccentric closure of the blastopore. — However, it cannot 
be denied that the gastrulation in Vertebrates exhibits 
certain peculiarities which call for an explanation. The blas- 
topore does not close in a concentric way. This is the case 
also in Annelids. The originally wide blastopore here closes by 
fusion of the two opposite lateral borders, leaving open only 
the foremost end which passes into the cardiac pore. 
How the blastopore border while contracting moves over the 
surface of the egg is not easily to be determined in Vertebrates 
Since we have no fixed landmark. Observing the living 
egg of a frog in its natural position, in which at first 
the animal pole is directed right upwards, gives no reliable 
results since during the gastrulation the centre of gravity 
of the egg changes its position and causes a rotation of 
the latter. In observing the egg in “Zwangslage” between 
two glass-plates and making e.g. photographs of it, for 
