308 APPENDIX A 



It is scarcely possible to suppose that either the compression of 

 the egg" or the gravitation plane brings the spermatozoa round to 

 the side of compression, but it may be imagined that either by 

 capillarity or by some chemotactic stimulus the spermatozoa are 

 especially attracted to the point where the rapidly swelling coats 

 of adjacent eggs come into contact, and that therefore fertilization 

 is principally effected upon this side. This explains why the first 

 furrow lies so often in this direction. The pressure may of course 

 affect the position of the planes in the egg later on. 



When the eggs are spaced the sperm enters on any side at 

 random. 



The deviation of the sperm entrance from the egg-axis (the 

 angle between sperm-entrance radius and egg-axis) varies in the 

 two series of observations. When the eggs are spaced and the 

 axes vertical, the sperm enters mainly near the equator, never 

 near the animal pole ; when the eggs are compressed and the axis 

 horizontal, usually at about 45 from the axis, though it may 

 enter near the pole or near the equator. This difference obviously 

 depends on the difference in the initial position of the eggs on 

 the slide. The deviation has apparently very little effect on any 

 of the planes we have been considering. 



Finally, let us try and gain some conception of the mechanism 

 by which the direction of the furrow depends on the point of sperm 

 entry. It is apparently quite simple, for the sperm-path is 

 directed usually towards the axis, the sperm nucleus travels along 

 that path to meet the female nucleus, which is also in the axis, 

 the centrosome of the sperm divides at right angles to that path, 

 the fertilization spindle is developed between the diverging 

 centrosomes and cell-division takes place in the equator of the 

 spindle ; the first furrow includes therefore the sperm-path. 

 Should, however, the ' penetration ' path not be exactly radial, 

 for whatever reason, the sperm nucleus turns aside to meet the 

 female pronucleus, there is a ' copulation', as distinct from a 

 ' penetration ' path, the centrosome divides at right angles to 

 the former, and this, then, is included in or parallel to the plane 

 of the furrow. In those cases in which the sperm-path is parallel 

 to the furrow it is always quite close to it, and we may suppose 

 perhaps that the first division has not been quite equal. (The 

 division of the centrosomes has not, I believe, been observed in 

 the Frog, and the foregoing description has been taken from the 

 Axolotl. In this genus the definitive centrosome is formed from 

 the sperm nucleus, when the latter has already penetrated some 

 little way into the egg.) 



The causes of the formation of the grey crescent which marks 

 the symmetry plane are not so clear. 



