J. W. Jbnkinson 
201 
towards the Sperm-path, while from what has been ascertained of the behaviour 
of chromatophores in the presence of light, we may also understand why this 
agent should modify the position of the grey crescent. 
The union of the pronuclei is, however, a process independent of the direction 
of first entrance of the sperm, and of the formation of the Sperm-sphere, in 
which the vacuoles are developed before the two nuclei meet. Here then are two 
perfectly distinct processes, on one of which the direction of cell division depends, 
while the other determines the s^'mmetry of the egg, with which is correlated the 
symmetry of the embryo. One of these can be readily affected by gravity and by 
light, the other cannot. But when all external disturbing influences are removed, 
when the Sperm-path is radial and the egg-nucleus is axial, then the causes which 
determine the symmetry of segmentation and those on which the symmetry of 
egg and embryo depend will be coincident. 
Experimentally however they may be separated from one another. This is, 
indeed, no new discovery for the Frog's egg, for Pflliger showed that in forcibly 
inverted ova, while the First Furrow might make any angle with the plane 
including the egg-axis and the vertical (the " streaming meridian " of Born, or, as 
I have called it, the gravitation symmetry plane), the dorsal lip of the blastopore 
always appeared in this plane and on the side on which the white pole was 
turned up. 
What is, I believe, new, is that the thirty minutes that pass before the egg 
turns over are sufficient for gravity to do its work. And this, I may remark, will 
explain the controversy that has been carried on for so long between Roux, 
Morgan and Kathariner on the one side, and Schulze and Moszcowski on the 
other. Roux maintained that the position of the First Furrow was determined 
not by gravity, but by causes residing in the structure of the egg. Schulze replied 
that gravity was a necessity for normal development, while Moszcowski claimed 
that the grey crescent, on which, as admitted and asserted by Roux, the position 
of the First Furrow, and later of the Sagittal Plane, depended, was brought about 
by gravity, quite a short time being amply sufficient for the purpose. It may be 
regarded as quite certain that the eggs kept in any constant state of slow rotatory 
motion (by being placed on a vertical wheel or in any other way) will produce 
normal tadpoles; and with regard to the second point, Morgan and Kathariner 
have shown that eggs kept in motion during insemination and later, still develope 
the grey crescent. This experiment is paralleled by placing the eggs on the slide 
with the axis vertical ; in them the crescent appears. But it is interesting to 
notice that though gravity is certainly not indispensable, it may still modify the 
position of the Symmetry Plane. 
The grey crescent of the Frog's egg is no solitary example of the change of 
structure that may be brought about by fertilization. The same conversion of a 
radial into a bilateral symmetry may be observed in other cases, and the same 
determination of the First Furrow by the point of entrance of the spermatozoon. 
Biometrika vii 26 
