40 CELL-DIVISION AND GROWTH II. i 



Again, Roux observed that Frogs' eggs sucked up into a tube 

 with a narrow bore became elongated either parallel or trans- 

 verse to the length of the tube, the axis of the egg lying in each 

 case lengthways. In the first case the division was at right 

 angles to, in the second usually parallel to, the tube in accord- 

 ance with the rule; but exceptionally, in the transversely 

 stretched eggs, the division was not perpendicular to, but co- 

 incided with, the extent of the greatest protoplasmic mass. 



However important a factor the disposition of the yolk may 

 thus be in deciding the direction of cell-division, it is certainly 

 not the only factor. In the eggs pressed between horizontal 

 plates there are many an infinite number of directions of 

 least resistance. In one of these the segmentation spindle elon- 

 gates, and at right angles to this the first furrow falls. This is 

 probably determined as it is determined in the normal Frog 

 and Sea-urchin egg by the point of entrance of the spermato- 

 zoon, or at least by the direction of the sperm-path in the egg. 

 The second division is at right angles to the first, and here the 

 direction may very possibly be decided on Hertwig's principle. 

 But why, in the next phase, should the furrows be at right 

 angles to the second rather than to the first, for the extent of the 

 protoplasmic mass is as great in each of the four cells, in a direc- 

 tion parallel to the first as to the second furrow ? Here, it is 

 clear, some other reason must be found for this succession of 

 divisions at right angles to one another. The cause is probably 

 to be sought for in the direction of division of the centrosomes ; 

 for these divide frequently soon after the telophase at right 

 angles to the axis of the previous figure. We thus gain a new 

 expression for Sachs' Law. 



The original direction of divergence of the centrosomes is, 

 however, by no means always the ultimate one, for the growing 

 spindle may be twisted out of its original position. Conklin has 

 made a careful study of this phenomenon in Crepidula, in which 

 egg he finds that vortical movements are set up in the cytoplasm by 

 the escape of nuclear sap at the beginning of mitosis. The move- 

 ments are in opposite directions in sister cells, centre in the spindle 

 poles, and often carry both nucleus and spindle into a fresh place. 

 These currents, which had been noticed previously by other 

 observers (by Mark in Limax and by lijima in Nefihetis), may 



