NUCLEAR DIVISION WITHOUT CELL DIVISION. 169 



egg. Actual fusion does not occur in this solution ; the cells 

 become perfectly spherical, touching only at points of their sur- 

 faces indicating an increase of surface-tension but without 

 fusing. 



In sea-urchin eggs similar phenomena are observed. In gen- 

 eral fusion takes place more slowly and less completely in these 

 eggs than in those of Asterias. Arbacia eggs placed in m-urea 

 solution during the 2- and 4-cell stages exhibit fusion in a large 

 proportion of instances ; within thirty minutes after transfer all 

 stages ranging through various dumb-bell-shaped compounds to 

 completely rounded fusion-products are usually seen. Fusions 

 occur also in later stages (16- and 32-cells) in the same manner 

 as in Asterias. The blastomeres exhibit a similar tendency to 

 the adoption of a spherical shape before fusion, indicating a pre- 

 liminary increase of surface-tension. 



Fusion occurs also in m-glycerine solutions ; separate blasto- 

 meres gradually assume the rounded form, and in a few instances 

 they fuse, but fusion is slower and less complete than in urea- 

 solutions. The glycerine acts destructively on these eggs as on 

 those of Asterias, leading before long to a swelling and granular 

 disintegration. 



III. THEORETICAL. 



From the above experiments the conclusion may be drawn 

 that cleavage, as well as the maintenance of the cleaved condition, 

 is dependent upon the presence of electrolytes in the cytoplasm. 

 The activity of these electrolytes must be regarded as due to the 

 ions into which they dissociate. The exact nature of the role 

 played by the ions is at present largely a matter of conjecture ; 

 but it is reasonable to infer that here, as in other relations, they 

 act chiefly by virtue of the electrical charges which they carry. 

 If this is admitted the following possibilities present themselves 

 for consideration. 



If the essential feature of cleavage is an enlargement of the 

 egg-surface due to diminution of surface-tension over certain 

 areas, the possibility at once suggests itself that this alteration 

 of tension may be dependent upon certain known electrical in- 

 fluences, to be more explicit, upon the appearance of a difference 

 of electrical potential at the surface of contact between the egg 



