214 L. V. HEILBEUNN 



In order to make the tables as brief and compact as possible, 

 many less important observations have been omitted. In the 

 third column, in indicating the number of times the centrifuge 

 handle was turned and the number of seconds consumed in the 

 process, the number of turns is given first and this is followed by 

 the number of seconds. Thus ''30 in 25" indicates that the 

 handle was turned thirty times in twenty-five seconds. In each 

 case the turning is assumed to have been uniform unless other- 

 wise stated. In the fourth column, the extent of the hyaline 

 zone, and, in many cases, the degree of distinctness with which it 

 appeared, is recorded. In referring to the hyaline zone, the ab- 

 breviation 'H.Z.' is sometimes employed; more generally, however, 

 it is omitted, the remarks being then understood to pertain to 

 the hyaline zone. The various fractions in the fourth column 

 are indices of the extent of the hyaline zone. A line passing per- 

 pendicularly to the zones of the centrifuged eggs and through 

 the egg center is regarded as the axis of stratification. The 

 fraction of this line which is included by the hyaline zone is 

 taken as an index of the extent of this zone. 



The temperature of the water containing eggs was recorded 

 both at the beginning and at the end of each experiment. As 

 every one knows, the length of time necessary for cleavage varies 

 with the temperature. In the second column is recorded the 

 time when cleavage began. Of course, there is always some vari- 

 ation among the eggs of a given batch. Usually a very few eggs 

 start ahead of the others. A minute or two later, 5 to 10 per cent 

 of the eggs can be observed segmenting, and in another minute 

 about 40 per cent will be dividing. The cleavage time indicated 

 in the tables is the time when about 5 to 10 per cent of the eggs 

 have begun to divide. 



In order to correlate the viscosity tests with the mitotic changes 

 occurring within the egg during the tests, the eggs were observed 

 as closely as possible. The Arbacia egg is unfortunately rather 

 opaque, and comparatively little cytological detail can be ob- 

 served in the living egg. After fertilization, the first indication 

 of mitosis is a long, clear streak of hyaline material. This streak 

 is probably the profile view of the disc which Fol described in 1877, 



