13 
vestigation of only one or a few forms — without ever 
considering the same processes in Invertebrates and 
evidently regarding the germinal layers as a mere histo- 
logical conception. 
Truly, there is often a histological or, better perhaps, a 
cytological difference between the cells of the primary 
entoderm and those of the primary ectoderm, resulting from 
a difference in the amount of yolk and well distinguished 
from the histological difference of the tissues to which they 
give rise in the adult form. However considerable this 
cytological difference may sometimes be,and however early - 
in development it may become evident, it is yet only 
of secondary significance in determining what we have 
to call ento- and ectoderm. The first criterion in those 
early stages is, as the names indicate: what disappears from 
the surface by invagination or delamination and what 
remains on the outside? Though in several Evertebrates 
with determinate cleavage we can already determine in the 
blastula-stage with perfect certainty which cells will become 
ento- and which ectoderm (and even which mesoderm), we 
yet are not allowed by this circumstance to call the blastula 
a gastrula, as has been done by some authors in the case 
of Vertebrates. In the same way it is sometimes possible 
in the gastrula to indicate in the two primary germlayers 
the cells which will become the mesoderm. This is 
neither a reason to deny that we have to deal in such a 
case with a gastrula. It is only the process of the topo- 
graphical separation by which the germinal layers originate. 
No doubt the great cytological difference often prevailing 
between ecto- and entoderm has developed phylogenetically 
Only after the topographical opposition, though in ontogeny 
it often becomes apparent before the latter. 
This has often been lost sight of in determining what 
we have to understand by gastrulation and by ecto- and 
entoderm in Vertebrates. In the lower forms such as Acrania, 
Cyclostomata and Amphibians the process of invagination is 
easily recognized. The difference in the cytological character 
of ecto- and entoderm-cells is sometimes less, sometimes 
more, evident, though as a rule not excessive. In Amphibians it 
IS more pronounced than in Amphioxus and, as a conse- 
quence, the process of invagination sometimes becomes 
less evident, especially in the more yolk-laden eggs. Never- 
theless, by means of the less yolk-laden eggs of other 
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