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first two cleavage planes) is a little shorter when measured 
dorsally than when measured ventrally (the two ventral 
cells in stage 4 being slightly larger than the two dorsal 
ones), it follows that the closure of the blastopore takes 
place exactly at the vegetative pole. 
If now we consider the eggs marked at b, c‚, or d, which are 
the points of intersection on the third or equatorial cleavage 
furrow, we find that these marks during the gastrulation process 
remain almost stationary. Their distance from the mark 
a at the animal pole certainly increases, but the difference 
is only very slight. When the dorsal blastopore lip is 
formed, the mark at b is found lying about just as far in 
front of it as in the eight-celled stage the point b is distant 
from the egg-equator. The more the blastopore lip is then 
shifted backwards the greater the distance becomes. Finally 
the mark is found on the medullary plate exactly behind 
the cerebral plate. From this it is quite evident that there 
can be no question about the whole rudiment of the embryo 
being formed by concrescense of the lateral borders of 
the blastopore, since the greater part originates in situ in 
front of the dorsal rim. The rudiment of the cerebral 
plate is situated from the beginning between the marks a 
and b. That there would be an invagination of animal 
cells round the dorsal blastopore border to invest the 
archenteron roof is rendered improbable by other pricking 
experiments in which marks were made slightly in front 
of the crescentic border as it was just appearing. In some 
cases truly | found this mark to approach and to reach 
the blastopore border, but soon the eggs on which l worked, 
which were of Rana esculenta, proved to develop abnor- 
mally and to show a tendency to “spina bifida,” ie. a 
retarded closure of the blastopore. In these eggs the dorsal 
blastopore border, instead of moving backwards, moves 
forward and in this case an invagination of cells round 
it seems to take place. In normally developing eggs, 
however, a mark, however closely it lies in front of the dorsal 
blastopore border, does not approach the latter, which 
shows that there occurs no invagination of cells from the 
Surface, as postulated by LWOFF. 
he mark at c is found again some distance before the 
anterior end- of the cerebral plate, in front of (properly 
speaking behind) the border of the so-called sense-plate 
which lies round the front part of the cerebral plate in 
