12 PHYSIOLOGY. 



The form of the nucleus is different in various cells. Usually 

 it is a round or oval body situated in the middle of the cell. Its 

 -rounded form is considerably expanded in young cells, as in the 

 ovaries in their evolution. Very frequently the form of the element 

 influences that of the nucleus. Thus, in muscle- and nerve- cells the 

 nucleus is generally elongated. In the lower organisms it sometimes 

 assumes the shape of a horseshoe or a twisted strand, or is very much 

 branched, the processes running out in every direction into the sur- 

 rounding protoplasm. 



The size of the nucleus is usually in proportion to the mass of 

 protoplasm enveloping it. Thus, in the large ganglion-cells of the 

 spinal cord the nuclei are correspondingly large. Also in cells en- 

 gaged in active work> the nuclei are generally of good size, as in the 

 secreting cells of the salivary and mucous glands. 



As to the number of nuclei present in a cell, the general condition 

 of the presence of but one in a cell seems to prevail. There are ex- 

 ceptions, however, as liver-cells very frequently contain two, aJid the 

 immense cells of bone-marrow, many. 



General Substance, or Structure. 



The nucleus is no more of a homogeneous nature than the proto- 

 plasm and presents several distinct substances and structures. The 

 different constituents that are known are not always present in all 

 cells, at all times, or in the same proportions. Among some cells one 

 element may be very conspicuous, while in some others it is scarcely 

 to be found. According to Verworn, the following substances occur 

 most constantly : (1) nuclear sap, (2) achromatic nuclear substance, 

 (3) chromatic nuclear substance, and (4) the nucleolus. 



The nuclear sap may be present in larger or smaller quantities 

 and is the liquid ground-substance which fills up the interstices left 

 among the solid nuclear constituents. In many cells under the in- 

 fluence of certain reagents, and even in life, it is known to be of a 

 very fine granular nature. 



The achromatic nuclear substance is a structure of fine threads 

 found in the nuclear sap, and it is characterized, as is also the latter, 

 by not staining with the usual reagents; carmin, hsematoxylin, etc. 

 It contains achromatin or limn. 



Lantanin is found in linin in the form of fine granules, which 

 stain by acid anilin dyes, as opposed to chromatin, which takes up 

 only basic anilin dyes. Hence lantanin is called oxychromatin, 

 whilst chromatin is known as basichromatin. 



