EARLY DEVELOPMENT OF THE FROG. 51 



the egg has become bilaterally symmetrical, with the plane of symmetry 

 cutting the center of the animal pole, the center of the vegetal pole and the 

 point of entrance of the sperm. There is also a visible external change in 

 the distribution of pigment. On the side of the egg opposite the point where 

 the sperm entered, some of the pigment granules over a crescent-shaped area 

 at the lower border of the pigmented surface are carried from their original 

 position, leaving this area lighter in color. The name, gray crescent, is 

 given to the lighter area which extends more than half way round the egg 

 (Fig. 27). 



The rearrangement of the egg substances disturbs the center of gravity of 

 the egg. The original axis, extending from the center of the animal pole to 

 the center of the vegetal pole, is inclined at an angle of about 30 degrees to 

 the vertical, the margin of the highly pigmented pole being tilted accordingly 

 out of the horizontal. The gray crescent lies on the higher side. The verti- 

 cal axis of the egg is now the gravitational axis, and, from the manner in 

 which the internal rearrangement of egg substances has presumably occurred, 

 a gravitational plane will bisect the egg into symmetrical halves, bisecting the 

 gray crescent and containing both the gravitational axis and the original 

 polar axis. All these changes have been caused or at least initiated by the 

 sperm. 



Cleavage. When the sperm nucleus reaches the egg nucleus via the copu- 

 lation path the two nuclei join to form the single nucleus of the fertilized 

 ovum. The sperm centrosome divides into two which take positions at 

 opposite poles of the single nucleus. A spindle develops between the cen- 

 trosomes, and the chromosomes assemble in the equatorial plane of the 

 spindle. The direction that the spindle assumes does not appear to be wholly 

 a matter of chance. In the first place it forms at right angles to the egg axis; 

 for it is generally true that the spindle of a cell in division lies in the direc- 

 tion of the greatest cytoplasmic mass. If the egg is not subjected to pres- 

 sure, the spindle tends to lie in the plane of egg symmetry or at right angles 

 to it, although there may be many variations. If there is pressure from 

 without, the spindle tends to lie at right angles to the direction of pressure. 

 The factors other than pressure which influence the direction of the spindle 

 have not been determined; but it appears that the spindle has a tendency at 

 least, to assume a position of symmetry relative to the structure or internal 

 organization of the egg. This means therefore that the first cleavage plane, 

 which of course cuts the spindle at right angles, tends to divide the egg in or 

 near the plane of symmetry or at right angles to it. In about 25 per cent, 

 of instances the first cleavage plane deviates but little from the plane of egg 

 symmetry; in about 10 per cent, it lies transversely to the plane of symme- 

 try. It is also true that the first cleavage plane tends to coincide with the 



