STUDY OF CELL MECHANICS 453 



protoplasmic streaming and the division is never permanent. 

 Following the second division cycle, a third sets in. The move- 

 ment of the second period is usually not completed by this 

 time. The movements of the third period are even more severe, 

 as a rule, than any of the earlier ones, and an endless number 

 of bizarre shapes are seen. One very characteristic type seen 

 is shown in figure 10. On such an egg little protoplasm drop- 

 lets will be thrown off which do not fuse again with the mother 

 egg, probably, because of the ectoplasmic layer has disintegrated 

 by this time. Several more division cycles may follow (Wilson, 

 '01 a, saw as many as 6 in one egg) but ultimtely disintegration 

 results (fig. 9). 



The behavior both of the aster and of the protoplasm, as has 

 been described in the foregoing pages, may be taken as typical 

 of what we would find, in any given set of monaster eggs. But 

 extreme variations are often found from the courses described 

 above. The figures together with the descriptions in the fol- 

 lowing section will give some idea of the extent of these varia- 

 tions and will show that they are variations in degree and not 

 in kind. 



In figures Bi to B 6 , the history of an egg is given in which 

 the aster did not move to any appreciable degree. The egg was 

 isolated when the controls were in the two-cell stage and it was 

 a typical monaster (fig. Bi drawn at 11.20). The egg under- 

 went little change for the first ten minutes or so, then little 

 chromosome vesicles began to appear and this was followed by 

 a slight swelling of the ectoplasmic layer (fig. B 2 , 11.42). This 

 was accompanied by a slight change in the contour of the egg's 

 surface. At 11.50, the rays of the aster had disappeared but 

 the protoplasm of the egg was still somewhat irregular in out- 

 line (fig. B 3 ). At 12.08, an amphiaster was found in the egg. 

 This lead to a division during which the binding membrane was 

 greatly swollen in the division plane and seemed to undergo a 

 sort of disintegration, as may be seen by figure B 5 . The separa- 

 tion of the blastomeres was complete and was accompanied by 

 the giving way of the cell wall, as was described for the eggs 

 shown in figures 7 and 8. At 2.12, this egg had divided several 



