II. I 



CELL-DIVISION 



and it is comparatively rare for a cell to disobey the rule. The 

 rule is, however, no universal law of cell-division. Every em- 

 bryologist will recollect the continued division of a teloblast in 

 the same direction to form a germ-band, which is such a con- 

 spicuous fact in the development of Molluscs, Annelids, and 

 Arthropods. The four polar nuclei of Insect eggs, lying in one 

 straight line, may also be cited. 



The direction of division and the size of the blastomeres are 

 not, however, the only factors which determine the actual pattern 

 of segmentation. The cells can, and do, shift their positions on 

 one another. This is of common occurrence, and a few examples 

 will suffice. The rearrangement of the tetrahedrally disposed 

 cells in Asteroids and Ophiuroids has been noticed already. In 

 many ' spiral ' ova the micronieres have been observed to rotate 

 on the macromeres, or one quartette to be pushed out of position 

 by the cells of another. In Ascaris the cell P 2 slips to one side. 

 Further, cells change their shape. 



Two factors are therefore involved in the production of the 

 pattern of cleavage, the direction of division, and the movements 

 of the cells, and these factors in their turn demand explanation. 

 To these must be added the shape, 

 the size, and the rate of division 

 of the cells. 



The two latter depend very 

 largely upon the amount of yolk 

 present in the egg ; yolk-cells are 

 large, the yolk divides slowly, or 

 not at all. This was expressed long 

 since by Balfour in the formula, 

 ' The velocity of segmentation in 

 any part of the ovum is, roughly 

 speaking, proportional to the con- 

 centration of the protoplasm there; 

 and the size of the segments is 

 inversely proportional to the con- 

 centration of the protoplasm.' J The rule has been vindicated by 

 O. Hertwig experimentally. If the egg of the Frog be centri- 



FIG. 15. Segmentation of the 

 Frog's egg under the influence 

 of a centrifugal force (from Kor 

 schelt and Heider, after 0. Hert- 

 wig). The egg consists of a blas- 

 toderm and an undivided yolk 

 (yolk-syncytium) : Teh, blastocoel ; 

 m, yolk-nuclei ; d, yolk. 



Comp. Emb. i. c. 3. 



