26 



FUNDAMENTALS OF CYTOLOGY 



stage," an obviously inappropriate term. At this stage the nucleus is 

 bounded by a membrane. The permeability of this membrane is known 

 to differ in certain respects from that of the plasma membrane: the two 

 are not formed between the same pair of substances. Although various 

 substances must be interchanged through the membrane, the cytolysis 

 following its tearing by a needle shows that it separates substances capa- 

 ble of strong interaction. 



Within the membrane is a mass of clear nuclear sap, or karyolymph , in 

 which the remaining nuclear constituents are embedded. The most 

 important of these constituents is evidently a substance in the form of 

 numerous crooked threads, the chromoncmafa (= color threads). These 



Fig. 13. — Nerve cell from earth- 

 worm, showing the fine neurofibrils within 

 the cytoplasm. {After J. Kowalski.) 



Fig. 14. — Nuclei in young floral axis 

 of Maianthemum. In the narrow cells of 

 the developing vascular bundle (right) the 

 nuclei become greatly elongated. Young 

 plastids are present in the cytoplasm. 



are so named because they contain a substance {chromatin) which is 

 strongly stainable with certain dyes and can thus be made to stand out 

 plainly in the clear, unstained karyolymph. In living nuclei they are 

 only faintly seen or even may be invisible because their refractive index in 

 ordinary light is so nearly like that of the karyolymph, but suitable meth- 

 ods reveal their presence (Fig. 15). Their visibility va&y vary with 

 alterations in the degree of hydration during the nuclear cycle. They 

 frequently appear, especially in fixed material, to be connected by fine 

 strands (anastomoses), forming thus a network (reticulum). The extent 

 to which anastomoses are normal structures is uncertain. The chro- 

 monemata are of special interest because they represent the chromosomes 

 at this stage. In stages of nuclear division an additional stainable sub- 

 stance (matrix) is combined with the chromonemata, giving the chromo- 

 somes the compact form characteristic of those stages (Chap. VII). 



