i6 



HISTOLOGY. 



entiate, die, and are cast off. Since they can produce only epidermal cells, 

 they are themselves partly differentiated. The fertilized ovum which can 

 produce all kinds of cells must be regarded, in spite of its size and great 

 mass of yolk-laden cytoplasm, as the least differentiated. 



The progressive specialization of cells concerns chiefly their proto- 

 plasm, yet in the case of the muscle fibers of the salamander it is accom- 

 panied also by marked nuclear changes. Typical muscle nuclei from 

 Necturus embryos 7 mm. and 26 mm. long, respectively, are shown in Fig. 

 1 8. The significance of the differences between them is not known, as 



they have been but recently de- 

 tected. The cytoplasm of muscle 

 cells differentiates its contractile 

 function beyond all others, and 

 becomes filled with contractile 

 fibrils. Many kinds of cells are 

 specially modified for producing 

 secretions which may either be 

 discharged, as from gland cells, 

 or in a somewhat solid state 

 may remain in contact with the 

 cell, thus forming certain of the 

 intercellular substances. Small 

 amounts of structureless intercel- 

 lular substance, such as is some- 

 times found between epithelial 

 cells, are called cement substance, 

 even though it may be fluid. 

 Between connective tissue cells 

 the intercellular substances are 

 formed in such quantity that they 

 far exceed the bulk of the cells 

 which produced them. These 

 ground substances may be homogeneous, or permeated with fibrils and 

 granules, formed either by the exoplasm or by the transformation of 

 the intercellular substance. The remnant of ground substance between 

 the fibrils is another so-called cement substance. In cartilage and bone, 

 the cells appear scattered through the ground substance which by their 

 differentiation they have produced. 



Regression or degeneration is the manifestation of approaching death. 

 Normally it is not seen in nerve cells and probably not in the voluntary 

 muscle cells. Subtle and unrecognized changes may occur in them in 



B 



FIG. 18. NUCLEI OF STRIATED MUSCLE FIBERS PROM 

 YOUNG SALAMANDERS (NECTURUS). (Eycleshy- 

 mer.) 



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

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

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



