236 L. V. HEILBRUNN 



June 28th. Eggs fertilized at 11:31 a.m., were at 11 :46| a.m.. dropped 

 into Stender dish B, which contained 40 cc. sea-water plus 10 cc. dis- 

 tilled water. The untreated fertilized eggs remained in Stender dish A . 

 At 11:51| A.M., eggs in A and B were centrifuged simultaneously, the 

 handle being turned 45 times in 26 seconds. The eggs in A showed 

 just the beginnings of a hyaline zone. The eggs in B were markedly 

 more stratified, they showed gray cap and hyaline zone plainly. In 

 this experiment the sea-water was not sufficiently dilute to prevent 

 segmentation. In another experiment it was found that a solution 

 made up of equal parts of sea-water and distilled water was the most 

 favorable for the reversible prevention of cleavage. In such a solution, 

 eggs remained unsegmented, and yet after a three-hour exposure, they 

 were able to resume their development when returned to normal sea- 

 water. 



SUMMARY 



1. During the period between fertilization and the first cleav- 

 age of the sea-urchin egg, the viscosity of the cytoplasm rises 

 until it reaches a maximum, then it decreases again. 



2. Similar viscosity changes occur in, relation to the second 

 cleavage. 



3. The changes in viscosity are very marked and indicate the 

 occurrence of. a gelation in the cytoplasm. 



4. This gel-formation reaches its height just before the spindle 

 appears. Later the cytoplasm becomes more fluid again. 



5. That gelation is not secondary, but is a predetermining fac- 

 tor in spindle or aster formation, is indicated by the fact that 

 when gelation is suppressed, the mitotic figure does not form, 

 although the eggs may be otherwise uninjured. 



6. Such suppression of gel formation was produced by four- 

 teen different substances, all lipoid solvents. 



7. It can also be produced by cold. 



8. Although they produce the same effect, the action of cold 

 and of lipoid solvents is mutually antagonistic. 



9. The effect of hypertonic solutions on dividing eggs can be 

 interpreted on the basis of the fact that they increase the cyto- 

 plasmic viscosity. Potassium cyanide and chloretone also act 

 in this way. 



10. The cytoplasmic gelation which occurs in relation to mito- 

 sis is apparently due to an abstraction of water, for — 



