198 Symmetry of Egg and Symmetry of Embryo in the Frog 
There is on the other hand but a very sliglit relation between the meridian 
occupied by either the entrance point or the inner end of the Sperm-path and the 
Plane of Symmetry, even under the most favourable conditions, and when the 
position of the Symmetry Plane is altered, as we know it can be altered, by 
gravitation, the correlation is negligible. A correlation of considerable magnitude 
has, however, been found between the whole (" penetration ") path of the sperm and 
the position of the Plane of Symmetry, and since the inclination of the Sperm- 
radius and Sperm-path have, by themselves, but little elfect, the only possible 
conclusion is that if the position of the grey crescent is dependent upon the 
spermatozoon at all, it must be determined by all the meridians successively 
occupied by the sperm as it travels from the surface into the interior. 
In this case it is evident that while gravity improves the relation between 
Sperm -path and First Furrow it utterly destroys the relation between Sperm-path 
and Plane of Symmetry. This is in exact accord with the results previously 
obtained. For it was shown that while gravity, during the short interval in which 
it operated, could very largely determine the position of the Plane of Symmetry, 
it had but little influence on the direction of the Furrow ; while there was a con- 
siderable tendency for the Plane of Symmetry to lie in the Gravitation Plane, 
there was but a slight tendency for the Furrow to be situated not in but at right 
angles to that plane. 
The internal factors which determine the direction of the deviation of cell 
division must be therefore distinct from those which determine the symmetry of 
the egg, and later, of the embryo. 
Let us briefly enquire what the mechanisms may be by which these two 
different processes are brought about. 
As far as the division of the fertilized ovum is concerned, it is not hard to 
understand why the Furrow should include the Sperm-entrance and path, or at 
least be parallel to the inner portion of the latter. The cell-division takes place 
in the equator of the fertilization spindle, where the spindle fibres thicken to form 
a cell-plate after the chromosomes have passed to opposite poles ; this equatorial 
plane is of course at right angles to the spindle axis — the line joining the two 
centrosomes, or, at an earlier stage, the direction of separation of the two halves 
of the divided sperm-centrosome (Fig. 4 D). 
This line lies in a plane parallel to the egg equator (probably owing to the 
manner of distribution of the yolk about the axis) and is at right angles to that 
uniting the male and female pronuclei, or " copulation " path of Roux. 
The first part of the path (" penetration " path of Roux) is simply the entrance 
funnel ; its formation is probably a capillary phenomenon, attributable to the 
aggregation about the acrosome of a watery substance withdrawn from the egg 
cytoplasm, and to the streaming of this substance into the interior of the egg 
(Fig. 4 A). The spermatozoon or sperm nucleus previously carried in by this 
movement then turns towards the female pronucleus, and this second portion of 
