506 
able to continue and repeat these experiments, as the number of eggs 
which in each of the cases reached the medullary plate stage without 
losing their marks did not exceed three or four. But the results of 
these agreed so completely that for the present my conclusion is that 
the marks at 4, c, or d are not appreciably displaced. 
When the dorsal blastopore lip is formed, the mark at / is found 
lying about equally far in front of it as in stage 8 the point / 
is distant from the egg equator. The more the blastopore lip is 
then shifted backwards the larger the distance becomes. At last the 
mark is found on the medullary plate exactly behind the cerebral plate. 
The mark at c is found back at 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 the shape of a crescent and the border 
of which (Fig. 2: *), as further development sbows, indicates 
the border of the head as far as the gill arches. On this plate the 
two suckers, the mouth and the two olfactory grooves are afterwards 
found. Behind this plate lie on both sides of the anterior end of 
the medullary plate (behind the cerebral plate) two little projections (4), 
representing the rudiment of the gill arches. In this vicinity the mark 
d is found back of which I have not been able to fix the place 
with great accuracy since it was exactly in these eggs that the 
marks were lost when the medullary plate began to form. | 
could state however that these too do not appreciably move away 
from the animal pole, but that a small displacement seemed to take 
place in the direction of 6, probably caused by the forming of 
the medullary plate and the accompanying thickening of the epithelium. 
This shows that the third or equatorial cleavage in the egg of 
Rana fusca fairly well separates the head and the trunk, at any rate 
as far as the ectoderm is concerned. The rudiment of the head, taken 
as far as the gill slits, we find therefore in the four upper small 
blastomeres of the eight-celled stage, that of the trunk in the four 
large lower ones. Moreover it appears that the rudiment of the cere- 
bral plate in stage 8 is found in the two smallest blastomeres of that 
stage, namely in the two animal and dorsal ones, each situated 
between the points a, 6, and d and that these two blastomeres 
probably do not produce much more than just the cerebral plate. 
The two other animal blastomeres, the ventral ones, furnish the 
so-called sense-plate, i.e. the ectoderm of the head. Very attractive, 
also when viewing the pictures, is here the supposition that this 
sense-plate represents the remainder of the episphere of the 
trochophora. In this case the four animal cells of the eight-celled 
