NUCLEAR DIVISION WITHOUT CELL-DIVISION. 165 



Both of the above classes of phenomena present the following 

 general resemblances : the surface-area of the entire egg first of 

 all undergoes a marked increase ; increase of area is accompa- 

 nied by corresponding alterations in the spherical form of the 

 egg ; an increase of surface-extent is thus normal to cleavage 

 and indeed may with justification be regarded as one of its most 

 essential features. 1 The increased surface-area does not in the 

 above instances remain unaltered, or increase still further as 

 normally, but undergoes a secondary and abnormal decrease, 

 seen in the smoothing of the incipient cleavage-furrows or the 

 refusion of the already separate blastomeres. These alterations 

 in surface-area are almost undoubtedly to be ascribed to altera- 

 tions in the surface-tension of the egg. The primary increase of 

 surface seen in cleavage thus indicates a general lowering of the 

 normal surface-tension ; this in the above instances appears to be 

 followed by an increase of surface-tension which leads to the 

 succeeding regressive series of changes. 



The problem of cleavage thus resolves itself, partially at least 

 if not entirely, into the problem of the nature of the condi- 

 tions producing alterations of surface-tension at certain definite 

 regions of the egg-surface. It is believed that the experiments 

 about to be described throw light upon this problem. They 

 have demonstrated that cytoplasmic cleavage is prevented in so- 

 lutions of non-electrolytes, although nuclear division continues ; 

 and also that a strong tendency to fusion makes its appearance 

 in the blastomeres of eggs transferred to such solutions in early 

 cleavage-stages. Since under these conditions the electrolytes 

 normally present in the egg must diffuse outward into the sur- 

 rounding medium, it is to be inferred that cleavage is closely 

 dependent upon the presence of electrolytes within the egg- 

 protoplasm. The manner in which ions derived from the dis- 

 sociation of the contained electrolytes may conceivably effect 

 alterations in the surface-tension of the egg will be considered in 

 some detail below. 



II. EXPERIMENTAL. 



The following experiments were performed during the past 

 summer at Wood's Roll as part of a series on which I was then 



1 Compare Rhumbler : Merkel und Bonnet's Ergebnisse, 8, 1898, p. 605 et seq. 



