340 CAUSES OF VARIATION 



equal parts ; (2) each new plane of division tends to intersect 

 the preceding one at a right angle. 1 



A typical cleavage of a spherical egg would be, first, vertical, 

 dividing into right and left halves ; second, also vertical, but at 

 right angles to the first, dividing into dorsal and ventral portions ; 

 third, horizontal, dividing into anterior and posterior parts ; 

 after which all sorts of irregularities might be expected, including 

 cleavages parallel with the surface, cutting in two the long cells 

 that formerly extended to the center. The egg of the holothu- 

 rian, like those of Synapta, proceeds regularly until as many as 

 512 cells are reached, 2 while others become irregular almost at 

 once, some quadrants dividing much more rapidly than others. 



Variation in the rate of division. If division proceeds with 

 perfect regularity, the number of cells will of course form an 

 increasing geometrical series whose ratio is two, 2, 4, 8, 16, 

 32, 64, 128, 256, 512, 1024, etc. Such a series has been men- 

 .tioned, extending to 512. 



This uniform series is rarely realized, however, owing to irreg- 

 ularities in the rate of division ; for example, Nereis regularly 

 gives rise to the series 2, 4, 8, 16, 20, 23, 29, 32, 37, 38, 41, 42, 

 " after which the order is more or less variable." 3 



In some portions of the dividing cell the cleavage proceeds 

 therefore with much greater rapidity than in others, nor is the plan 

 uniform in all cases, though the results achieved may be sub- 

 stantially identical. In general the rate of division is most rapid 

 in the upper hemisphere of the ovum, and in some instances it pro- 

 ceeds very slowly, with long pauses, in the lower, giving great 

 irregularity in the size of cells. 



Temporary effect of outside influences upon cleavage. Driesch 

 placed eggs of sea urchins under pressure sufficient to flatten the 

 spheres to disks. In this position "the amphiasters all assume 

 the position of least resistance, i.e. parallel to the flattened sides, 

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

 two cells. This is totally different from the normal form of 

 cleavage ; yet such eggs when released from pressure are capable 

 of development and give rise to normal embryos." 4 Without 



1 Wilson, The Cell, p. 362. Ibid. p. 366. 



2 Ibid. p. 364. 4 Ibid. p. 375. Italics are mine. 



