„10 
had made here an angle of 45° with the symmetry-plane of the 
egg. Perhaps a couple of strong mutually perpendicular cross- 
lines (“Brechungsfurchen”) at the animal and vegetative poles 
were also a result of this. Now I made a small mark on the 
equatorial cleavage furrow over the highest point of the “grey 
field”, i.e. halfway between two points of intersection which may 
best be denoted by 4 and d, although in such a case this nomenclature 
does not hold, of course. The result was what might be expected 
according to Bracner: the mark behaved entirely as a mark 
made at 4 in a “typical” egg. The blastopore lip was formed 
—— and contracted right under it 
(fig. 3) and finally lay on the 
ea 
is ii middle of the medullary plate, 
just behind the cerebral plate. 
Hence the symmetry-plane of 
the embryo coincides with that 
of the unsegmented egg, inde- 
pendent of the direction of the 
first two cleavages. I hope very 
much that future years may 
ig Ye. offer an opportunity for conti- 
: nuing and extending these expe- 
EN riments, which are a first at- 
Fie. 3 tempt to attain at something 
For explanation see the text like cell-lineage investigations 
Little mark right above the crescent- in vertebrates. Also on eggs of 
shaped blastopore border. Rana esculenta I made some 
preliminary experiments, the results of which are very interesting 
and will be published shortly. 
For the present it may be stated that between the directions of 
the first two cleavage planes and those of the main axes of 
the embryo no direct and constant relation exists. Such a 
relation does exist, however, between their line of intersection, the 
main axis of the egg, with which the longitudal axis of the 
embryo (as long as the tail has not begun to develop) coincides. 
This also holds for the third cleavage plane which approximately 
separates head and trunk and to which accordingly a greater pospective 
significance must be attributed than to the former two. 
LITERATURE. 
AssHeTon, R., 1894, On the Growth in Length of the Frog Embryo. Quart. 
Journ. Vol, 37. 
