GEOMETRICAL RELATIONS OF CLEAVAGE-FORMS 2?$ 



fact 1 that the immediate cause of the inequality probably does not 

 lie either in the nucleus or in the amphiaster ; for not only the 

 chromatin-halves, but also the asters, are exactly equal in the early 

 prophases; and the inequality of the asters only appears as the 

 division proceeds. Probably, therefore, the cause lies in some rela- 

 tion between the mitotic figure and the cell-body in which it lies. 

 I believe there is reason to accept the conclusion that this relation 

 is one of position, however caused. A central position of the mitotic 

 figure results in an equal division ; an eccentric position caused by a 

 radial movement of the mitotic figure, in the direction of its axis 

 towards the periphery, leads to unequal division, and the greater the 

 eccentricity, the greater the inequality, an extreme form being beauti- 

 fully shown in the formation of the polar bodies. Here the original 

 amphiaster is perfectly symmetrical, with the asters of equal size 

 (Fig. 71, A). As the spindle rotates into its radial position and 

 approaches the periphery, the development of the outer aster be- 

 comes, as it were, suppressed, while the central aster becomes enor- 

 mously large. The size of the aster, in other words, depends upon the 

 extent of the cytoplasmic area that falls wit kin the sphere of influence 

 of tJic centrosome ; and this area depends upon the position of the 

 centrosome. If, therefore, the polar amphiaster could be artificially 

 prevented from moving to its peripheral position, the egg would 

 probably divide equally. 



The causes that determine the position of the amphiaster are 

 scarcely known. It has been proved by experiment that in some 

 cases this position may be determined by mechanical causes. Thus, 

 Driesch has shown that when the eggs of sea-urchins are flattened 

 by pressure, the amphiasters all assume the position of least resist- 

 ance, i.e. parallel to the flattened sides, so that the cleavages are all 

 vertical, and the egg segments as a flat plate of eight, sixteen, or 

 thirty-two cells (Fig. 135). This is totally different from the normal 

 iorm of cleavage ; yet such eggs, when released from pressure, are 

 capable of development and give rise to normal embryos. This 

 interesting experiment makes it highly probable that the disc-like 

 cleavage of meroblastic eggs, like that of the squid or bird, is a 

 mechanical result of the accumulation of yolk by which the forma- 

 tive protoplasmic region of the ovum is reduced to a thin layer at 

 the upper pole ; and it indicates, further, that the unequal cleavage 

 of less modified telolecithal eggs, like those of the frog or snail, are 

 in like manner due to the displacement of the mitotic figures towards 

 the upper pole. Even here, however, the hypothesis of a merely 

 mechanical displacement probably does not touch the root of the 



1 In the cleavage of gasteropod eggs. 



