212 L. V. HEILBRUNN 



VISCOSITY CHANGES DURING MITOSIS 



During the process of mitosis the cytoplasm of the sea-urchin 

 egg undergoes marked physical changes. I have already shown 

 (Heilbrunn, '15) that a gelation occurs soon after mitosis, so 

 that the early stages of the mitotic process are associated with a 

 gradual stiffening of the egg protoplasm. In later stages this 

 gelation is reversed, so that in the anaphase the egg has again 

 returned to its original fluid condition. 



In order to follow closely the physical changes which occur in 

 the cytoplasm, viscosity measurements were made by the centri- 

 fuge method. An accurate measure of the cytoplasmic viscosity 

 can be obtained by a determination of the ease with which gran- 

 ules move through it. When the cytoplasm is quite fluid, a 

 moderate centrifugal force is sufficient to throw all the granules 

 into one half of the egg, so that a clear 'hyaline zone' extends 

 through the other half (except for a small polar accumulation of 

 lipoids, the 'gray cap'). As the viscosity of the cytoplasm in- 

 creases, the granules cannot respond so readily to centrifugal 

 force, and the hyaline zone, instead of extending through half of 

 the egg, only appears in a limited portion. With still greater 

 increase in viscosity, the same or even a stronger centrifugal force 

 causes no perceptible movement of the granules. The width of 

 the hyaline zone is not so easy to determine in the fertilized egg 

 as in the unfertilized egg, for soon after fertilization the pigment 

 granules characteristic of the Arbacia egg all migrate to the cor- 

 tex, and since they are not so readily moved when in this position, 

 they remain as an outer coat completely surrounding and par- 

 tially obscuring the interior cytoplasm. But with a little prac- 

 tice this difficulty is easily overcome by proper focusing, and it 

 becomes a simple matter to estimate the width of the hyaline 

 zone. So great are the changes in its extent that it is sufficient 

 to express its width approximately in terms of the egg diameter. 

 In this way rapid determinations can be made and viscosity 

 tests can be completed in a very short time. Thus, in a number 

 of experiments I was able to determine the egg viscosity at in- 

 tervals of about four minutes. 



