509 
I then tried to show that the polar body in Amphioxus would 
be situated at a considerable distance before the nenropore. Now it 
has turned out, however, that also in Craniota the animal pole does 
not come on, but in front of the cerebral plate. Does not this invalidate 
my former reasoning? In no way. If the reader will take the trouble 
to compare the two pictures of Amphioxus embryos reproduced in 
that paper, he will perceive at once that there can be no question 
that in Amphioxus the polar body will obtain a place corre- 
sponding to a mark on the animal pole of the frog egg, i.e. exactly 
in front of the neuropore. The distance from the neuropore is so large 
that it entirely agrees with the ideas then put forth by me and 
which are moreover supported by strong anatomical arguments, 
according to which the fore-brain proper of the Craniota is 
lacking in Amphioxus. As well in Annelida as in Acrania and 
Craniota we find that the animal pole finally lies approximately on 
the foremost point of the prostomium and therefore also of the body. 
Finally a single experiment may still be mentioned, which I hope 
to extend later, but which already can confirm a conclusion lately 
reached by Bracner in a different way. On the fertilised but still 
unsegmented frog egg a bilateral symmetry is soon observed, caused by 
the white — in the form of the so-called grey field (Roux) — 
extending on one side, the dorsal one, higher up towards the egg 
equator than on the other sides. In the majority of cases the first 
cleavage plane coincides with the plane of symmetry of the unsegmented 
egg and the symmetry-plane of the embryo is then in its turn the 
same as these two. Such eggs with “typical” development (Roux) 
I always selected, as was stated above, for the marks a, 5, c, or d. 
Variations are not very rare, however, the first cleavage plane 
sometimes making a more or less considerable angle with the symmetry- 
plane of the egg, which angle may amount to 90° (anachronism of 
the two first cleavages). Now Bracuet by killing one of the first 
two blastomeres by means of a hot needle and by studying the so 
formed hemi-embryos, arrived at the conclusion that in such cases, 
in which the symmetry-plane of the egg and the first cleavage plane 
do not coincide, the svmmetry-plane of the embryo corresponds to 
that of the egg and is independent of the direction of the first 
cleavage plane. This conclusion is confirmed by what follows. 
I came across an egg in the eight-celled stage in which the 
highest point of the white field did not lie on one of the two vertical 
cleavage furrows, as is tne case with typical cleavage, but halfway 
between the two, henee not under point 4, but halfway between 
4 and one of the ds. Evidently the first and also the second division 
