270 CELL-DIVISION AND DEVELOPMENT 



{^^6) and Errara ('86, '87) have pointed out that in almost all cases 

 the cells tend to alternate or interlock so as to reduce the contact-area 

 to a minimum. Thus arise many of the most frequent modifications 

 of cleavage. Sometimes, as in Synapta, the alternation of the cells is 

 effected through displacement of the blastomeres after their forma- 

 tion. 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 posi- 

 tion of least resistance from the beginning. This form of cleavage, 

 especially characteristic of many worms and mollusks, is typically 

 shown by the ^gg of Polygordius (Fig. 121). The four-celled stage is 

 nearly like that of Synapta, though even here the cells slightly inter- 

 lock. 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 like- 

 wise oblique, but at right angles to the third, so that all of the cells 

 interlock as shown in Fig. 121, D. This alternation regularly recurs 

 in the later cleavages. 



This form of cleavage beautifully illustrates Sachs's second law 

 operating under modified conditions, and the conclusion is irresistible 

 that the modification is at bottom a result of the same forces as those 

 operating in the case of soap-bubbles. 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 ogg, 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. 122). It is a curious fact 

 that the direction of the displacement is extremely constant, the 

 upper quartet in the eight-cell stage being rotated in all but a few 

 cases to the right, or with the hands of a watch. Crampton ('94) has 

 discovered the remarkable fact that in Physa^ a gasteropod having a 

 reversed or sinistral shell, the whole order of displacement is likewise 

 reversed. 



The third class of modifications, due to unequal division of the cells, 

 leads to the most extreme types of cleavage. Such divisions appear 

 sooner or later in all forms of cleavage, the perfect equality so long 

 maintained in Synapta being a rare phenomenon. The period at 

 which the inequality first appears varies greatly in different forms. 

 In Polygordius (Fig. 121) the first marked inequality appears at the 

 fifth cleavage; in sea-urchins it appears at the fourth (Fig. 3); in 

 AinpJiioxns at the third (Fig. 123); in the tunicate Clavelina at the 

 second (Fig. 126); in Nereis at the first division (Figs. 43, 122). The 



