THE NUCLEUS 53 



The limitations of this method are not altogether clear, but it has already- 

 proved valuable in distinguishing chromosomal matter from other sub- 

 stances which stain like it with certain dyes. Most investigators have 

 found that vital dyes do not stain the nucleus as long as it is living and 

 healthy, but P. Dangeard (1923a) reports that nuclei in certain cells with 

 a somewhat retarded activity may be so stained with neutral red and 

 methyl violet. Successful results are reported by several other workers. ^^ 



Structure of the Nucleus. — The typical nucleus is bounded by 

 a nuclear membrane. The reality of this membrane has often been 

 questioned, but researches on living cells^^ leave no doubt that it is a 

 definite structure. It is usually very thin; it has even been suggested 

 that it may sometimes be but a single layer of molecules. In most cells 

 under the micromanipulator it appears, however, to be thicker, remaining 

 intact when the nucleus is pushed and pulled about or removed from the 

 cytoplasm, and crumpling into folds when fluid is withdrawn from the 

 nucleus with a pipette. Its origin with respect to the regions which it 

 separates is obscure. The suggestion has been made that it is double, 

 having distinct nuclear and cytoplasmic components. ^^ 



The larger portion of the volume of most nuclei is composed of a 

 highly transparent ground substance known as karyolymph, or "nuclear 

 sap." This usually appears optically homogeneous, with a few" dispersed 

 particles at most. The results of microdissection and the Brownian 

 movement of suspended particles show that it may be in either the sol 

 or the gel state. In old cells vacuole-like masses may appear within it 

 (Kisser, 1922). 



Associated with the karyolymph is another substance which appears 

 in most preparations as a more or less continuous network of threads, the 

 reticulum. The naturalness of this structure has also been a subject of 

 debate. Many living nuclei, notably in animals, appear perfectly 

 homogeneous (excepting the nucleolus) or at most very finely granular, 

 a reticulum appearing gradually as the nuclei are injured experimentally 

 or allowed to become moribund. ^^ The coarseness and the uniformity 

 of the reticulum often vary considerably with the fixing agent employed ; 

 moreover, the coagulation of colloidal substances very commonly involves 

 the formation of fibrous and reticulate structures. It has accordingly 

 been inferred that the thready reticulum has no real existence in the 

 normal living state but represents an artifact brought about by altera- 

 tions in the nuclear colloids. 



i^Kuster (19186, 1926), Gicklhorn (1927), Paltauf (1928), Albach (1928). The 

 reactions of plant and animal cells to vital dyes are compared by Benoist et al. (1929). 



^* Kite, Chambers, C. V. Taylor, Gross, Nemec. 



i^Scarth (1927), Klingstedt (1928). 



"Lewis and Lewis (1924), Chambers (1924), Schaede (1925, 1930), Strugger 

 (1930). 



