172 Symmetrn of Egg and Symmetry of Emhryo in the Frog 
also towards the point in the axis occupied by the female pronucleus, and in all 
cases it was held that it was the line of union of the two pronuclei which deter- 
mined the position of the (meridional) cell division. The pronuclei are situated 
in the equator of the fertilization spindle, and the spindle lies in a plane parallel 
to the equator of the egg. The direction of the axis of the spindle, and therefore 
of its equator, is however really determined, as many cytological studies have 
shown, by the dii'ection of division of the sperm-centrosome. Since this, however, 
is at right angles to the line joining the two pronuclei and in a plane parallel to 
the equator of the egg the result is the same. 
Roux further fertilized the eggs from an arbitrarily selected meridian, by 
applying the sperm containing water to one side of the egg by means of a fine 
cannulus, a camel's hair brush or a fine silk thread. The fertilization meridian so 
selected became the meridian of the First Furrow and, as Roux believed to be 
necessarily the case, the Median Plane of the embryo. Lastly, Roux believed that 
the grey-crescent appeared always on the side opposite to the entrance of the 
spermatozoon, and was in fact caused by that entrance, and that the Sagittal Plane 
lay in the Plane of Symmetry so produced in the egg. Since, moreover, the dorsal 
lip appeared upon the side of the grey-crescent, the entrance point of spermatozoon 
marked the future ventral side. With this plane the First Furrow — the plane of 
the predetermined or 'immanent' qualitative division of the segmentation nucleus 
— was also held to coincide. In short, this one cause was supposed to determine the 
Symmetry of the unsegmented egg, the Symmetry of Segmentation, and the 
Symmetry of the Embryo. 
The subsequent statements of O. Schulze and Kopsch with regard to the sperm 
entrance, grey-crescent. First Furrow and Sagittal Plane are in substantial agree- 
ment with Roux's. Since, however, as we now know, the causes which determine 
the position of the First Furrow can be experimentally separated from those which 
determine the position of the other two planes, and are therefore presumably 
distinct, it has seemed to me to be worth while enquiring how far and in what 
sense it can be asserted that one cause, the entrance of the spermatozoon, is 
responsible for both. 
I have accordingly examined by the help of sections the relation of the sperm- 
entrance point and sperm-path to the First Furrow in a number of eggs in which 
the angle between the Plane of Symmetry and the First Furrow had been 
previously ascertained. 
As in the series of experiments already described the eggs were taken from the 
uterus and placed upon glass slides, fertilized, covered with water and left till the 
First Furrow appeared. The angles between the Plane of Symmetry and First 
Furrow and the lines ruled across the slide were then measured and recorded. 
Each egg was given a number, and killed by means of boiling water containing a 
little formol and preserved separately, being later cut into a series of sections. The 
eggs were placed on the slide under two different conditions. In one series they 
