LIVING SUBSTANCE 91 



of one nucleus correspond exactly to those of another. Neverthe- 

 less, a number of constituents, which apparently are wide-spread, 

 are even now fairly well characterised. The following substances 

 occur most constantly : 



1. The nuclear sap constitutes the liquid ground-substance, in 

 which the solid nuclear constituents are contained (Fig. 32). M. 

 Heidenhain, Reinke, and Korschelt have lately demonstrated that 

 in many cells, even during life, it presents an extremely finely 

 granular appearance. 



2. The achromatic nuclear substance forms in the ground-substance 

 a supporting-structure of fine threads, which are characterised, like 

 the nuclear sap in which they are suspended, by not staining with 

 the typical nuclear stains, such as the carmine stains, haemotoxylin, 

 etc. 



3. The chromatic nuclear substance is distinguished from the 

 achromatic by its property of staining with these reagents. It is 

 contained in the strands of the achromatic substance, as a rule in 

 the form of small granules and irregular particles, and upon its 



FIG. 32. Various nuclei from the mother-cells of the spermatozoa of the thread- worm of the 



horse. (After Hertwig.) 



staining-power chiefly rests our knowledge of the finer structure of 

 the nucleus. 



4. The nucleolus is a homogeneous granule which is present 

 comparatively rarely in nuclei ; it consists of a strongly refractive 

 substance which appears to be closely related to the chromatic 

 substance. Since, as a rule, the substance of the nucleoli may be 

 stained by the nuclear stains like the chromatic substance, the 

 nucleolus has been considered by many investigators as a special 

 accumulation of chromatic substance a view which, however, 

 because of the different relations of the two substances toward 

 certain chemical reagents, cannot strictly be maintained. 



All of these substances, to which with advancing knowledge of 

 the nucleus others will perhaps be added, are present in very dif- 

 ferent quantities in different cells. A substance that is abundant 

 in one nucleus may be insignificant in another, and it even appears 

 as if certain substances can be wholly wanting in certain nuclei. 

 In many cases the nuclear substances are surrounded and marked 

 off from the protoplasm by a special nuclear membrane, which, how- 

 ever, like the cell-membrane in relation to the cell, is not a general 

 constituent of the nucleus. 



