CELL-DIVISION AND GROWTH 



II. i 



for instance, in the egg of Echinoids after the expulsion of the 

 polar bodies and before fertilization. Apart from such excep- 

 tions, due very likely to some temporary alteration in the relations 

 of yolk and cytoplasm, the rule is a reliable one. 



The relation between the dividing nucleus, the spindle and 

 centrosomes and the cytoplasm has been stated by O. Hertwig 

 in his second empirical rule ' that the two poles of the division 

 figure come to lie in the direction of the greatest protoplasmic 

 mass ', by Pfluger in the formula, ' the dividing nucleus elongates 

 in the direction of least resistance.' 



The objection that has been urged to this latter expression, 

 that in a fluid the pressure is equal in all directions, may be 

 set aside. For though the cytoplasm is fluid it is an extremely 

 viscid fluid, and the presence of the suspended yolk granules 



FIG. 16. Diagram ot the segmentation of the Frog's egg (after 

 O. Hertwig, from Korschelt and Heider). A, first (meridional) ; B, third 

 (latitudinal) phase of segmentation ; p, superficial pigment of animal 

 hemisphere ; pi; protoplasm ; y, yolk ; sp, spindle. 



must certainly offer a greater resistance than the fluid itself, 

 and greater in proportion to their number and size. Pfluger's 

 formula, therefore, if not merely a truism, resolves itself into 

 a restatement of Hertwig's rule. This rule certainly holds 

 good for a large number of cases, for it explains, for instance, 

 the two first meridional divisions of all spherical telolecithal 

 and radially segmenting eggs, the third, latitudinal (in small- 

 yolked eggs 1 ), and possibly also subsequent meridional and lati- 

 tudinal divisions (Fig. 16). It will not, however, in the present 

 state of our knowledge, explain the obliquity of the spindles to the 

 egg-axis in spirally dividing ova, nor cases of bilateral division ; 

 1 In Si/con the third is meridional, the fourth latitudinal. 



