2l6 FINE-STRUCTURE OF PROTOPLASM II 



the visible structure of fixed nuclei. On fixing, the fibrils of the nucleus 

 are dehydrated and become accessible to staining. They usually 

 clot together as a result of the adhesive action of the coagulated 

 proteins of the enchylema. 



The chain molecules, forming the fibrils in badly fixed nuclei, may 

 be so highly hydrated that no structure whatsoever can be detected 

 in the living nucleus (Pischinger, 1950). Such nuclei are homo- 

 geneous in the electron microscope when properly fixed (Rqzsa and 

 Wyckoff, 1950). These observations do not negative an amicroscopic 

 nuclear structure. It is possible that the chain molecules, although 

 completely hydrated, may be paralleli2ed in the same way as they are 

 known to be in cellulose solutions. In this state the nucleus is thixo- 

 tropic; and it may behave like a liquid drop, in which the nucleolus 

 falls to the bottom when observed in a horizontal microscope (Harris, 



1939)- 



The framework structure in the nucleus has received a much more 



appropriate name than in the cytoplasm, where the misleading concept 



of foam or honeycomb structure is often used. For it is designated 



as a reticulum, which clearly expresses that both framework substance 



and karyolymph are continuous structural components. In the living 



nucleus the threads of the reticulum are separate, but during fixation 



they coalesce and are held together by the coagulating protein of the 



enchylema. 



The living reticular framework is not rigid, but is to some extent 

 plastic. By means of centrifugal forces Nemec (1929) has displaced 

 the nucleolus in the nucleus, or even removed it altogether, in which 

 case the reticulum was deformed. According to several authors the 

 nucleus has the nature of a liquid (e.g., Schaede, 1927) or even no 

 structure at all. This is derived from deformability and optical homo- 

 geneity. The spherical shape, the capacity to form drops and the fact 

 that living nuclei are often optically empty are put forward as further 

 arguments. For this reason it is necessary to repeat that the behaviour 

 of a colloid, whether elastic like a gel or more, liquid like a sol, does 

 not in itself prove or disprove the existence of a submicroscopic 

 structure. To decide this, measurements of structural viscosity are 

 necessary, a property which the highly viscous nuclear substance pos- 

 sesses in a marked degree, as has been shown by Harris (1952). 



Since the structural elements of the nuclei are represented by the 



