438 L. V. HEILBRUNN 



however, that the time of cleavage is taken as the time of com- 

 pleted segmentation. The interval between viscosity increase 

 and the first constriction of the cell is somewhat less. Obviously 

 it is not much more than a minute. 



In the sea-urchin egg a similar viscosity increase occurs just 

 prior to division. This was indicated by some of my earlier 

 data (cf., for example, the second table on page 216 of my 1920 

 paper). In earlier experiments, however, I was primarily inter- 

 ested in the viscosity increase which precedes the appearance 

 of the spindle, and I did not study carefully the viscosity at 

 the conclusion of mitosis. The intervals between tests were 

 too large and, moreover, I took as the time of segmentation the 

 time when the first eggs began to divide. It is much more exact 

 to consider as the time of segmentation the moment when 50 

 per cent of the eggs have divided. 



In Arbacia there is a rise in viscosity before division. This 

 is shown conclusively by some tests made in 1919. In these 

 tests an old-model Bausch & Lomb centrifuge was used. It 

 differs from the new model in having a shorter distance between 

 the ends of the tubes, the radius of turn being 6 cm. 



June SO, 1919. Temperature 18.0° at 10.58 a.m., 19.0° at 11.42 a.m. 

 Arbacia eggs were fertilized at five-minute intervals. In finger-bowl 

 A eggs were fertilized at 10.55 a.m., in B at 11.00 a.m., and in C at 

 11.05 a.m. At 12.101 P.M. less than 1 per cent of the eggs in A had 

 begun segmentation. At 12.10f p.m. eggs from A and B were centri- 

 fuged simultaneously in separate tubes, the high-speed handle being 

 turned 30 times in 25 seconds. On examination of centrifuged eggs, 

 the great majority of eggs from A showed no hyaline zone, only about 

 10 to 20 per cent showing such a zone. The centrifuged eggs from 

 B all showed a hyaline zone plainly extending one-third of the distance 

 through the egg. 



At 12.17j P.M. about 5 per cent of the eggs in B were beginning to 

 segment. Starting at 12.17| p.m., eggs from B and C were centrifuged 

 simultaneously, the high-speed handle being turned 30 times in 28 

 seconds. When eggs from B were examined, there was no hyaline zone 

 in any of the eggs. Eggs from C all showed a hyaline zone, some- 

 times indistinct, sometimes extending a third of the distance through 

 the egg. 



Other tests gave similar results. Evidently just before divi- 

 sion in Arbacia as in Cumingia there is a sudden increase in 



