THE ESSENTIALS OF HISTOLOGY. 



FIG. 1. DIAGRAM OP A CELL. 

 toplasm composed of spongio- 



and hyaloplasm ; n, nucleus 



with intranuclear network, n' , and 



of spontaneous movement. When the cell is uninclosed by a membrane 

 a change in the shape, or even in the position of the cell, may be 

 thereby produced (amoeboid movement, see Lesson V.). The proto- 

 plasm often exhibits a granular appearance, which, under high 

 magnifying powers, is seen to be due to the fact that it is composed 

 of two distinct substances (fig. 1), one a reticulum or sponge work, 

 which appears under the microscope in the form of a network, and the 

 other a clear soft substance which occupies the interstices of the 

 reticulum, and may also cover the surface or project beyond the rest of 



the cell. The granular appearance above 

 mentioned is caused by the knots in the 



* 



network appearing when imperfectly ob- 

 served as separate granules. The material 

 which forms the reticulum is termed spong- 

 ioplasm ; the clear material which occupies 

 its meshes is hyaloplasm. The protoplasm 

 often includes actual granules of albu- 

 minous or fatty nature, or globules of 

 watery fluid (vacuoles) containing glycogen 



, ,, . , . ,_- ., 



or other substances in solution. Materials 

 which are thus included in the proto- 

 plasm of a cell are either stored up for the nutrition of the 

 cell itself, or are converted into substances which are eventually 

 extruded from the cell in order to serve some purpose useful to 

 the whole organism, such as the secretion which is furnished by 

 the cells of a gland. The term paraplasm has been given by 

 KupfFer to any such material within a cell other than the actual 

 protoplasm. Paraplasm is often present in sufficient amount to 

 reduce the protoplasm to a relatively small amount, the bulk of the cell 

 being occupied by other material, as when starch becomes collected 

 within vegetable cells or fat within the cells of adipose tissue. In some 

 cells there are fine but distinct stria3 or fibrils running in definite 

 directions. These are very commonly met with in fixed cells, such as 

 various kinds of epithelium-cells, nerve- and muscle-cells. But besides 

 this special differentiation, which appears to be related to the special 

 function of the cell, and is not universal, there is another definite 

 structure in the cell-protoplasm, which is known as the attraction-sphere 

 (fig. 2). This consists of a wheel-like arrangement of fine fibrils or rows 

 of granules, which radiate from a clear area, in the middle of which lies a 

 central particle the attraction-particle. The attraction-spheres were 

 discovered by v. Beneden in the ovum or egg-cell, and were at first 

 supposed to be peculiar to the ovum, but they have now been recog- 



