32 S. S. COHEN 



involving chromosomes, for the distribution of large numbers of genetic units. 

 Indeed, we should be at a total loss, temporarily at least, if the genetic units 

 present in interphase nuclei should prove not to be organized in a differ- 

 entiated thread plus an existing procentromere, i.e., point of spindle fiber 

 attachment, so necessary for the activities of the chromosome during mitosis. 



The experimental evidence for the presence of chromosomes in such 

 materials is not the strongest. It is considered that the inabihty to see such 

 structures arises from their imbibition of water, such that the refractive 

 index of the swollen and extended chromosome approaches that of the 

 surrounding nuclear sap. However, when needles have been placed within 

 resting nuclei and wiggled about, it has not been possible to detect the 

 presence of the anticipated resistant structures. On the other hand, it is 

 reported that the insertion of carbon particles into nuclei has resulted in an 

 apparent outlining of ghostlike chromosomes. The isolation of "chromo- 

 somes" and chromatin threads from resting nuclei has been described by some 

 workers, as will be described below; such reports have been questioned and 

 the products have been described as artifacts. 



Ris (1957) has stated that even in electron micrographs of sections through 

 the interphase nucleus "it is not at once obvious what is nuclear sap and 

 what chromosomal material." However, after an examination of sections of 

 cliromosomes, he feels that he knows what to look for and reports the pre- 

 sence of characteristic chromosome fibrils in the sections of interphase nuclei 

 of various plant and animal tissues. Whether such fibrils are truly character- 

 istic of chromosomal material alone and, indeed, reflect the presence and 

 organization of the genetic material in such nuclei wiU require far more study. 



"Heterochromatin" has been defined by some workers as chromosomal 

 regions that remain condensed and darkly staining throughout interphase. 

 The existence of such chromocenters in interphase nuclei are then presumably 

 markers of the existence of cliromosomes bearing these differentiated regions. 

 Ris (1957) has observed that sections through heterochromatin in isolated 

 lampbrush chromosomes and the chromocenters of interphase nuclei reveal 

 the same fibrillar structures that may be seen in the sections of the bulk of 

 the cliromosomes, the euchromatin. 



2. The Visualization of Intrayiuclear Constituents 



In the last 15 years, the cytologist concerned with the relationship of 

 visible structure to cell function has produced a revolution in cytological 

 technique. Abandoning a purely descriptive methodology, he has turned to 

 the chemist and physicist for collaborative assistance in developing quanti- 

 tative data for the characterization of cell structure. The ordinary 

 light microscope may resolve structures down to 1000 to 2000 A; phase 

 microscopy and ultraviolet microscopy have pushed this limit to about 



