LUBRICATING TISSUES 



391 



Mucous Lubricating Cells of the Epidermis of the Earthworm. The 



slime cells are well represented by the mucin-producing cell of the epi- 

 dermis of the earthworm. These cells may be considered as certain of 

 the simple epithelium cells that cover the surface of the body, and that 

 have been differentiated in structure so that they are able to secrete the 

 specific substance called mucin. The mucin is produced as good-sized, 

 hard granules in the cytoplasm of the cell, and has the property of mixing 

 with water into a jelly-like, viscid mass of many times its original size. 

 The substance can be identified easily in a number of ways by its staining 

 properties and by its solution reactions in different media. The granules 

 of mucin are produced in the cytoplasm of the cell and in regions of this 

 cytoplasm that are arranged in more or less straight rows reaching from 

 the proximal to the distal end of the cell. The substance of the mucin 

 comes from the blood as a fluid, invisible to the observer, and this fluid 

 is converted into the mucin by the activities of the cytoplasm in ways 

 that we cannot as yet understand. They appear as granules of a small 

 size and grow to their 

 full size, which is con- 

 siderable (Fig. 355). 

 When fully formed the 

 granules fill the entire 

 cell to such an extent 

 that it is distended to 

 many times its original 

 bulk. The reason of 

 this great distention is 

 that the cell matures 

 all of the mucin gran- 

 ules at once. This is 

 not true of some other 

 mucin-producing cells which contain the granules in all stages of 

 maturity, and which are constantly giving off some that are ripe. 

 No matter how distended the cell may become it never moves proxi- 

 mally out of the line. When the earthworm's cell is ripe it dis- 

 charges its contents in a very short time and is then seen in a state 

 of collapse. Some peculiar differentiated areas in the cytoplasm now 

 show the positions that were occupied by the granules, and the nucleus, 

 instead of being crowded down against the bottom of the cell and flat- 

 tened out, has arisen to a point about one fourth of the height of the cell 

 and has assumed a round contour and the characteristic chromatin and 

 nucleolar conditions of functional activity. The cell begins, after a very 

 short rest, to secrete mucin again. 



The mucin is extruded from the cell through the fine pore in the 



FIG. 355. Vertical section of a bit of epidermis of the earth- 

 worm. Shows four mucous cells in different stages of secre- 

 tion, mostly later stages. CM., cuticle with two pores shown, 

 from one of which mucus is emerging. X noo. 



