2 THE ESSENTIALS OF HISTOLOGY 



of fibres or cells. Some tissues are composed almost entirely of fibres 

 with relatively few cells interspersed amongst the fibres ; this is the 

 case with most of the connective tissues. Others are composed entirely 

 of cells, which, however, may in some cases be prolonged so as to form 

 fibres ; but these are different from the fibres of the connective tissues 

 in being formed directly from the cells of the tissue, whereas the fibres 

 of the connective tissues are formed between the cells in an intercellular 

 substance. Tissues which are entirely composed of cells are the epithe- 

 lial tissues, whilst nervous and muscular tissue are formed of cells 

 which are partly or wholly extended so as to form fibres. 



Cells. In the early embryo the whole body is an agglomeration 

 of cells. These are minute portions of living substance or protoplasm, 

 enclosing a vesicle which is known as the nucleus. The tissues are 

 subsequently formed either by changes which occur in the intercellular 

 substance, or by changes in the cells themselves ; frequently by both 

 these processes combined. The cells which are least altered from 

 their embryonic condition are the white corpuscles of the blood, and 

 these may be regarded therefore as typical cells. 



The protoplasm of a cell (fig. 1, p) is composed of a soft albuminous 

 substance, which is characterised in typical cells by possessing the 

 property of spontaneous movement. When the cell is unenclosed by a 

 membrane a change in the shape, or even in the position of the cell, 

 may be thereby produced (amoeboid movement). The protoplasm 

 often contains granules of various kinds or droplets of watery fluid 

 (vacuoles) ; the latter may be present in sufficient abundance to impart 

 a reticular or sponge-like structure to the protoplasm. In some cells 

 the protoplasm has a striated or fibrillar structure. 



The nucleus of the cell (fig. 1, n) is a minute vesicle embedded in 

 the protoplasm. It is bounded by a membrane which encloses a clear 

 substance (nuclear matrix), and the whole of 

 this substance is generally pervaded by an 

 irregular network of fibres, some coarser, 

 others finer (intranuclear network, n'). This 

 intranuclear network often exhibits one or 

 more enlargements, which are known as 

 the nucleoli. The fibres within the nucleus 

 have been observed to undergo spontaneous 

 changes of form and arrangement, but this 

 FIG. i. DIAGRAM OK A CELL, becomes much more evident when the cell 

 P 'Sr,nucie":-!tMn^ is about to divide. The division of the proto- 

 nnciear network, n'.aiid nucieo- pi asm j g always preceded by that of the 

 nucleus, and the intranuclear fibres undergo 



during its division a series of remarkable changes in arrangement 

 and position, which are known collectively by the term karyomitosis 

 (karyokinesis). These changes may best be studied in the division 

 of epithelium-cells (see Lesson VI.), but exactly similar phenomena 

 have been shown to occur in cells belonging to the other tissues. In 



