368 CELL-D/riS/OX .l.Vn DEVEI.Or.MRXT 



the most treiiucnt modifications of cleavage arise. Sometimes, as in 

 Syuapta, the alternation of the cells is effected through displacement 

 of the blastomeres after their formation. More commonly it arises 

 during the division of the cells, and may even be predetermined by 

 the position of the mitotic figures before the slightest external sign 

 of division. Thus arises that form of cleavage known as the spiral, 

 oblique, or alternating type, where the blastomeres interlock during 

 their formation and lie in the position of least resistance from the 

 beginning. This form of cleavage, especially characteristic of many 

 worms and mollusks, is typically shown by the egg of Polygordiiis 

 (Fig. 170). The four-celled stage is nearly like that of Synapta, 

 though even here the cells slightly interlock. The third division is, 

 however, oblique, the four upper cells being virtually rotated to the \ 

 right (with the hands of a watch) so as to alternate with the four 

 lower ones. The fourth cleavage is likewise oblique, but at right 

 angles to the third, so that all of the cells interlock as shown in 

 Fig. 170, D. This alternation regularly recurs for a considerable 

 period. 



In many worms and mollusks the obliquity of cleavage appears 

 still earlier, at the second cleavage, the four cells being so arranged 

 that two of them meet along a "cross-furrow" at the lower pole of 

 the &g^, while the other two meet at the upper pole along a similar, 

 though often shorter, cross-furrow at right angles to the lower {e.g. in 

 Nereis, Fig. 171). It is a curious fact that the direction of the dis- 

 placement is quite constant, the first or upper quartet in the eight- 

 cell stage being rotated to the right, or wdth the hands of a watch, 

 the second quartet to the left, the third to the right, and so on, 

 Crampton ('94) has discovered the remarkable fact that in P/iysa, a 

 gasteropod having a reversed or sinistral shell, the whole order of 

 displacement is likewise reversed, and the same has recently been 

 shown by Holmes ('99) to be true of Ancylns. 



The spiral or alternating type of cleavage beautifully illustrates 

 Sachs's second rule as affected by modifying conditions ; for, as may 

 be seen by an inspection of Figs. 170, 171, each division-plane is 

 approximately at right angles to the preceding and succeeding 

 (whence the " alternation of the spirals " described by students of 

 cell-lineage), while they are so directed that each cell as it is formed 

 is placed at once in the position of least resistance in the mass, i.e. in 

 the position of minimal surface-contact. It is impossible to resist 

 the conclusion that one of the factors by which the position of the 

 cells (and hence the direction of cell-division) is determined is a 

 purely mechanical one, identical with that which determines the 

 arrangement of soap-bubbles and the hke. 



Very little acquaintance with the facts of development is however 



