910 
movement of the dorsal rim. Such a primary wandering backward 
may be noticed in fact in such eggs, which to this end. have been 
provided with marks at a shorter or longer distance in front of and 
behind the blastopore border. Evidently it is the result of the forming 
of an invagination border at this place, where cells, lying originally 
eN 
Fig. 5. The same egg, from the side, 8 May, 1.50 pm. 
in front of the primary transverse rim of fig. 2 and 3, are carried 
inward. This however does not mean, that epiblast cells wander 
into the interior to participate in the construction of the archenteron 
roof. To me the view of Mac Bripr') seems to be preferable, 
according to which the first transverse slit does not appear at the 
border of the ecto- and entoderm area, but within the entoderm area, 
a little under the demarcation line. Thus the slit does not represent 
so much the first beginning of the blastoporic rim, as that of the 
archenterie invagination beneath it, and the cells in front of it, 
which disappear under the just forming blastoporic rim, are to be 
counted to the entoderm. Hence it is no wonder, that in a 
somewhat further advanced stage we find the blastoporic border a 
litle in front of the rim of fig. 2 and 3, which: is rendered the 
1) E. W. Mac Brive, 1909, The Formation of the Layers in Amphioxus etc. 
Quart. Journ. Vol. 54. 
