Cn. iy] CLEAVAGE OF THE EGG 39 



etc., it might no doubt segment irregularly, or it may be that 

 the motion equalizes the external conditions so that the eggs keep 

 a more nearly spherical shape and hence divide more regularly. 



Max Schultze ('63) and Rauber ('82) have made the most 

 careful study of the variations in the planes of division of the 

 fourth cleavage. In Figs. 13, D, F, G, and 14, B, C, are shown 

 the upper hemispheres of several eggs. If we examine the 

 position of the planes of the last (fourth) divisions, we see that 

 in the upper hemisphere each new cleavage-plane fails generally 

 to reach the black pole of the egg^ but passes to one or to the 

 other side. In the lower hemisphere the new planes fall far 

 short of the white pole. 



Occasionally we find eggs in the upper hemisphere of which 

 one or more of the fourth planes reach the black pole itself, 

 and, therefore, lie more nearly radial in position (Fig. 13, C). 

 In other cases, however, one or more of the new fourth cleav- 

 age-lines may even be nearly in the horizontal plane. In the 

 lower pole also there is much variation, and occasionally a blas- 

 tomere is divided into a very small and a very large part, owing 

 to the sudden turning aside of the new cleavage-line, so that 

 it meets one of the first two cleavage-planes before it lias 

 extended far into the lower hemisphere. Other eggs at this 

 same stage show a strictly bilateral arrangement of the cells 

 in the upper hemisphere. In Fig. 13, D, F, G, we see that 

 the fourth cleavage-planes have met the same furrow (first or 

 second). Also in Fig. 14, B, a bilateral symmetry is present, 

 formed in a somewhat different way, as the figure shows ; and 

 in this egg the lower hemisphere also is symmetrically divided. 



During the fifth cleavage-period the irregularities in the 

 division of the cells is generally so great that we cannot speak 

 definitely of any special direction of the new planes. Never- 

 theless there is a tendency for some of the new furrows to 

 come in at right angles to the last planes of division. There- 

 fore, many and occasionally all of the new fifth cleavage-planes 

 are horizontal (Fig. 12, C). The eight cells in the upper hemi- 

 sphere divide into equal or nearly equal parts, but the eight 

 blastomeres of the lower hemisphere divide unequally into eight 

 upper smaller blastomeres containing pigment, and eight lower 

 blastomeres which are the white blastomeres around the lower 



