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the path makes an angle with the first. Should the first portion have been 
directed towards the axis, and should the female pronucleus, as is usually the case, 
also lie in the axis, then both parts will lie in one atid the same meridional plane ; 
this becomes the plane of the First Furrow. But should the first part be inclined 
away from the axis, only the second will lie in a meridional plane, which becomes 
the plane of division for the same reasons. Should the divergence of the first part 
from the axis not have been great (in only three cases out of fiftj^-seven has it 
been found to exceed 30°, and in only one of these to be greater than 45°) the 
First Furrow may still practically include the point of entrance of the sperm. 
But there are cases in which the First Furrow deviates widely from the Sperm- 
path, even when the latter is directed towards it. The reason for this may very 
possibly be that the female pronucleus has not moved axially, but exaxially in its 
return from the animal pole ; the meridians of Sperm-entrance and " copulation " 
path, or in other words the meridians of Sperm-entrance and First Furrow, would 
then be more or less widely separate from one another. I have, however, no 
direct evidence that this is what has occurred ; but Roux states that in eggs 
taken at the end of the breeding season — after a long stay in the uterus — the 
female pronucleus is exaxial. The dates on which my eggs were taken were, for 
Series I. (close, horizontal), March 30th, for Series II. (spaced, vertical), April 4th. 
In Oxford the breeding-season ends usually in the third week of April. 
And now let us turn to the Plane of Symmetry. This is brought about by 
the formation of the grey crescent, and this is due, according to Roux, to the 
disappearance into the interior of some of the superficial pigment of the egg. 
The pigment disappears over a crescentic region at the border of the pigmented 
area on one side of the egg. When all the pigment has gone, the crescent is 
white and indistinguishable in colour from the original circular white area, though 
its position may of course be readily detected (Fig. 7). I do not, of course, know, 
but I cannot help supposing that this retreat of the pigment into the inside is 
directly due to the streaming movement set up in the egg-cytoplasm by the 
sperm. 
It is known from cytological descriptions that the entrance-funnel is an 
accumulation of a clear substance ; this takes place about the apical body or 
acrosome of the spermatozoon as soon as the latter gets inside the surface 
membrane of the egg ; it looks as though the clear substance were more watery 
than the surrounding cytoplasm, as though it were due to a local abstraction of 
water. Again, at the bottom of the entrance-funnel the Sperm-sphere, surrounded 
later by the Sperm-aster, is formed, and this consists of a yolk-free area of cyto- 
plasm (Fig. 5 C) in which later on large vacuoles of a clear, watery looking substance, 
become evident. Some of these vacuoles can be seen in the Sperm-sphere of the 
Axolotl figured in Fig. 5 D, and I have observed a precisely similar appearance on 
the inner side of the Sperm-nucleus in the Frog's egg. It may be supposed in 
like manner that these vacuoles are due to a local abstraction of water from the 
cytoplasm. 
