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species, the gastrulation in yolk-laden eggs may be easily 
traced back to the simple process in Amphioxus. 
A peculiar feature of the gastrulation in Vertebrates which 
has greatly contributed to the prevailing confusion of opinions 
is the eccentric way in which the border of the blastopore 
contracts to the final narrow opening that passes into 
the neurenteric canal. We shall revert to this. Firstly we 
must consider the questions: which stage represents the 
gastrula, what must we understand by the gastrulation 
and what must we call ento- and ectoderm? If, to answer 
these questions, we look to the Invertebrates for a comparison, 
especially to the Protostomia, and keep in mind the consider- 
ations given above, our conclusion must be: the gastrulation 
is the sinking away from the surface of part of the cells 
and the contraction of the blastopore-border over them. 
The gastrula, as a consequence, is the stage where 
the blastopore-border has contracted to a very narrow, often 
slit-like, opening that passes into the neurenteric canal in 
the same way as in Protostomia it passes into the corres- 
ponding cardiac pore, as 1 (1917b, p. 1267) have proposed 
to call the passage from the ectodermal stomodaeum into the 
entodermal gut. The ectoderm then, is what lies at the 
surface in this stage and the primary entoderm what lies 
in the interior, lining the archenteric cavity. 
Different views. — Several authors have come to other 
conclusions by paying more attention to the histological 
difference than to the topographical relation of the two 
primary germ-layers. The former, just as in Invertebrates, 
may become evident during the cleavage, long before 
gastrulation sets in. In the Amphibian egg eg. we can 
distinguish the future ecto- and entoderm by the colour and 
the size of the cells in early cleavage-stages, though _no 
sharp boundary between the two can be traced as yet. For 
Amphioxus the same holds good, though here the difference 
between the cells of both areas is less conspicuous and the 
transition from the one into the other equally gradual. 
This circumstance, together with the above mentioned 
peculiarity of the eccenfric blastopore-closure by which 
the gastrulation and the gastrula distinguish themselves 
from those of Invertebrates, has induced some authors to 
put forward the view that already in what we call the blaS- 
tula-stage and during the cleavage of the egg the differen- 
tiation of ecto- and entoderm is completed and that, if we 
