STUDY OF CELL MECHANICS 485 



SUMMARY 



1. A study of monaster eggs, living and in sections, has been 

 made. 



2. It has been shown that, at each division cycle, the egg 

 protoplasm is affected in three ways: (a) first there is a pro- 

 nounced swelling of the ectoplasmic layer, (b) this is followed 

 by intense changes in surface tension leading to the formation 

 of pseudopod-like processes, (c) and accompanied by a flow of 

 the superficial protoplasm toward the area where the streaming 

 is taking place. 



3. The intensity of the protoplasmic movement varies with 

 the position which the aster takes at the end of the division 

 cycle. 



4. A comparison shows that the behavior of monaster and 

 normal sea urchin eggs is similar. 



5. The changes in the egg protoplasm are preceded by definite 

 changes in the aster and in the chromosomes, and an analysis 

 of the behavior of the eggs is made to determine whether these 

 phenomena are due to the aster, nucleus, protoplasm, or to a 

 combination of these. 



6. No decisive evidence can be given to prove which part of 

 the cell is concerned, but many facts point to the nucleus. 



7. The swelling of the ectoplasmic layer may be produced 

 by treating eggs with hypertonic sea water. 



8. An attempt is made to eliminate the aster as a possible 

 source for the behavior of the monaster egg, by treating eggs 

 with phenyl urethane. 



9. The experiments show that, in the absence of asters, the 

 eggs may divide, the division being accompanied, in some cases, 

 by a swelling of the ectoplasmic layer in the cleavage plane and 

 a movement of the protoplasm into the cleavage' furrow. 



10. These facts eliminate the aster (and the processes which 

 presumably lead to their formation, centripetal flow of proto- 

 plasm, etc.) from the problem. 



11. All evidence points to the nucleus as the source of a sec- 

 ond factor in cell division, a factor (or factors) which finds its 

 expression in, a swelling of the ectoplasm, great changes in 



