Viscosity Changes of Protoplasm 185 



The first-known event is the migration of part of the centrosome 

 material to the opposite pole. This has not been seen or reported 

 for Uving fibroblasts, but undoubtedly occurs some time before 

 prophase. 

 Events leading to the formation of the daughter nuclei. 



The only visible structures in a resting nucleus are one to several 

 nucleoli, a homogeneous nucleoplasm, and a thin nuclear membrane. 

 In early prophase fine granules appear in the nucleoplasm. They 

 seem to become larger and larger and are recognizable as chromo- 

 somes toward mid-prophase. They ultimately almost fill the nucleus 

 except for a small amount of nuclear sap between them. During 

 the change from late prophase to metaphase the chromosomes 

 become more distinct and probably smaller and occupy less space 

 in what has now become the spindle than they did in the prophase 

 nucleus. The condition of the chromosomes in the resting nucleus 

 is obscure because they are invisible. It may be that they are 

 swollen and of low viscosity, and occupy the entire nucleus. Gelation 

 and then contraction of the chromosomes with a loss of fluid might 

 make them visible and also account for their small size as compared 

 with the relatively large amount of spindle material in metaphase 

 stage. 



The chromosomes split and move to the poles to form the two 

 small compact masses and leave behind a large amount of exnuclear 

 sap which extends across the midline from one mass to the other. 

 These nuclear masses soon begin to increase in size as clear areas 

 appear and increase in size and number. As the nucleus enlarges, 

 the visible chromosome material gradually becomes more or less 

 dispersed as granules. Most of them disappear, and the few that 

 remain unite to form one to several nucleoli. It may be that there 

 is a reversal of the prophase process in that the chromosomes 

 take up fluid, swell and solate, and become invisible except for the 

 nucleolar part. 



The nuclear membrane seems to disappear in late prophase just 

 before the chromosomes begin to move and does not become visible 

 until after the daughter nuclei begin to increase in size. It is very 

 thin and scarcely recognizable except as the sharp border of the 

 nucleus. It is probably in the gel state. Its fate is unknown. Does 

 it solate and disappear, or does it contribute to the spindle? 



The chromosomes suddenly begin to move in late prophase. The 

 movements are scarcely perceptible to the eye, but in motion pictures 

 where events are speeded up they become quite pronounced. They 



