IO 



HISTOLOGY 



tions. As a result of their own protoplasmic activity, the cells of many 

 tissues become surrounded by intercellular substances, which may far 

 exceed in bulk the cells which produced them. Intercellular substances 

 may be solid or fluid. When present in small amount they form thin 

 layers of cement substance between closely adjacent cells; in large amount 

 these substances constitute a ground work in which the cells are imbedded, 

 as, for example, in cartilage and bone. 



Although the differentiation of cells is chiefly cytoplasmic, there is 

 some evidence of corresponding nuclear changes. Thus while the muscle 



cells of the salamander are elabor- 

 ating complex fibrils, the nuclei 

 become modified as shown in Fig. 

 9. The significance of the nuclear 

 changes is unknown. 



Degeneration is the manifesta- 

 tion of the approaching death of 

 the cell. In nerve cells this process 

 normally takes place very slowly. 

 These cells remain active through- 

 out life, and if destroyed, they 

 can never be replaced. In many 

 glands, in the blood and in the 

 skin, however, the cells are con- 

 stantly dying and new ones are 

 being differentiated. In a few 

 organs the cells perish, but no new 

 ones form, so that the organ to 

 which they belong atrophies. 

 Thus a large part of the meso- 

 nephros (Wolffian body) disap- 

 pears during embryonic life; the 

 thymus becomes vestigial in the 



adult; and the ovary in later years loses its chief function through the 

 degeneration of its cells. 



The optical effects of degeneration cannot at present be properly classi- 

 fied. In a characteristic form, known as "cloudy swelling," the cell en- 

 larges, becoming pale and opaque. In another form the cell shrinks and 

 stains deeply, becoming either irregularly granular or homogeneous and 

 hyaline. The nucleus may disappear as if in solution (karyolysis, chroma- 

 tolysis); or it may become densely shrunken or pycnotic, and finally break 

 into fragments and be scattered through the protoplasm (karyorhexis) . 

 If the process of degeneration is slow, the cell ma/ divide by amitosis. 

 It may be able to receive nutriment which it cannot assimilate, and thus 



FIG. 9. NUCLEI OF STRIATED MUSCLE FIBERS FROM 

 YOUNG SALAMANDERS (NECTURUS). (Eycleshy- 

 mer.) 



A, From a 7 mm. embryo; B, from one of 26 mm.; ch, 

 chromatin knot; g. s, ground substance; 1, linin 

 fibril; n, nucleolus; n.m, nuclear membrane. 



