IV] 



STRUCTURE OF THE CELL 



189 



associated with surface-tension changes, of which the first step 

 is a minute puckering of the surface-skin, a sort of interdigi- 

 tation with the surrounding medium. For instance, Schewia- 

 kofE has observed in Euglypha* that, just before the break-up 

 of the nucleus, a system of rays appears, concentred about it, 

 but having nothing to do with the polar asters : and during the 

 existence of this striation, the nucleus enlarges very considerably, 

 evidently by imbibition of fluid from the surrounding protoplasm. 

 In short, diffusion is at work, hand in hand with, and as it were 

 in opposition to, the surface-tensions which define the nucleus. 



Fig. 58. Final stage in tfte first 

 segmentation of the egg of Cerebra- 

 tulus. (From Prenant, after Coe.)| 



Fig. 59. Diagram of field of force 

 with two similar poie.?. 



By diffusion, hand in hand with surface-tension, the alveoli of 

 the nuclear meshw^ork are formed, enlarged, and finally ruptured : 

 diffusion sets up the movements which give rise to the appearance 

 of rays, or striae, around the nucleus : and through increasing 

 diffusion, and weakening surface-tension, the rounded outline of 

 the nucleus finally disappears. 



* Schewiakoff, Ueber die karyokinetische Kerntheilung der Euglypha alveolata, 

 Morph. Jahrb. xm, pp. 193-258. 1888 (see p. 216). 



f Coe, W. R., Maturation and Fertihzation of the Egg of Cerebratuhis, Zool. 

 Jahrbiicher {Anat. Abth.), xii, pp. 425-476, 1899. 



