306 APPENDIX A 



gravitation, as it usually lies in the gravitation symmetry plane. 

 It is not, however, only so determined, for if the eggs (compressed 

 and with axis horizontal) be allowed to develop in the light the 

 plane of symmetry lies either in the gravitation symmetry plane, 

 or in the direction of the incident light (parallel to the length of 

 the slide in the experiment), while in the dark it lies only across 

 the slide. That this second effect is due to the light and not to 

 the pressure is shown by the fact that it occurs when the eggs 

 are spaced, and that it may be made to vary in position by 

 varying the position of the slide with regard to the light. 

 Light, therefore (ordinary daylight), as well as gravity, can help 

 to determine the position of the plane of symmetry, and when 

 the latter is excluded it appears that this plane is placed either 

 in or at right angles to the source of light. 



Light appears to exert no effect upon the first furrow. 



It is now intelligible why, when all these factors are operative, 

 the relation between the first furrow and the planes of symmetry 

 of egg and embryo should be disturbed, since, in the conditions 

 of the experiment, those factors which determine the position of 

 the former are at right angles to those on which the direction of 

 the latter depends. 



It still remains for us to inquire into the internal causes of 

 the direction of these planes in the egg. Roux, as has been 

 pointed out, has asserted that the grey crescent appears on the 

 opposite side of the egg to that on which the spermatozoon has 

 entered (pp. 80, 165), and further that the point of entry of the 

 sperm also determines the meridian of the first furrow, since this 

 either includes the sperm-path, or is parallel to it, or, when it is 

 crooked, includes or is parallel to the inner portion or ' copulation ' 

 path, which is taken to represent the line of approximation of 

 the two pronuclei; the outer part being simply the 'penetration' 

 path. Roux also arbitrarily selected a fertilization meridian 

 (meridian of the sperm-entry), and showed that this became 

 the ventral side (opposite the grey crescent) later on, as well as 

 the meridian of the first furrow (p. 248). 



I have been able to accurately investigate by means of 

 sections the relation between the fertilization meridian, first 

 furrow, and sperm-path in a number of eggs in which the 

 direction of the symmetry plane had been previously determined, 

 and the results of the measurements of these angles are given 

 here. The eggs fall into two series, those which were compressed 

 and had their axes horizontal (a), and those which were spaced 

 and had their axes vertical, the white pole being below (5). In 

 (a) the gravitation symmetry plane and the direction of com- 

 pression were at right angles to one another, as before. 



