VARIOUS KINDS OF CELLS: TISSUES 25 



is emphasized and in which the secretion which is produced is 

 of a fluid character. 



We must now pass to the consideration of the cells by which 

 contraction is carried out. In these, of course, a considerable 

 portion of the living substance has been modified into contrac- 

 tile fibres, the structure of which has already been described 

 (Chap. Ill) ; indeed these cells are often called muscular fibres. 

 But a muscular fibre and a muscular fibril (Fig. 2) are not 

 identical terms. A muscular fibre is a cell the cytoplasm of 

 which is usually made up of many fibrils, and in addition of a 

 certain amount of residual unmodified cytoplasm in which the 

 nucleus is situated. When the fibrils are plain or unstriped, the 

 cell is spindle-shaped and the nucleus remains single. When, 

 however, the fibrils are cross-striped, the fibre often becomes 



FlG. 4. Diagram showing gradual change of epithelial cells 

 into muscle cell /, the basal tails of the cells in which 

 contractile fibrils appear. 



enormously lengthened, and the nucleus divides and gives rise to 

 many daughter nuclei, which are scattered along the length of the 

 fibre (see Fig. 2), each embedded in a little nest of unmodified 

 protoplasm. Now it is a most interesting fact that as we 

 examine a large series of animals we can find every intermediate 

 stage between a muscular cell and an epithelial cell Thus in 

 certain sponges, which may be regarded as the lowest Metazoa, 

 the cells forming the outer epithelium contract when irritated. 

 When relaxed they are like flat tiles, but they gradually change 

 into the shape of pillars as they contract, but this contraction 

 is a slow process. If we ascend one step higher in the scale of 

 Metazoan animals, and examine the sea-anemones, we find that 

 in them the inner epithelium has the power of contraction, but 

 in this case we find that each contractile epithelial cell has its 

 base produced into a fibre, the cytoplasm in which is changed 

 into muscular fibrils. The power of contraction is confined to this 

 elongated base. Within the group of the sea-anemones every 



