EPITHELIUM 



45 



aligned to form an inner surface (Fig. 48, D) which later is provided with 

 a complete membrane that lies between it and the other body-cells lying 

 inside it. This is called the basement membrane, and differs much in the 

 amount of its development. It is not always present, and may be thick 

 and irregular or thin, tough, and smooth, and the cells sometimes pro- 

 ject through it into the tissue beneath. It provides openings for nerves 

 that come from the interior to end in the epithelium (see Fig. 192), and 

 for lymph channels, or even for blood vessels, when it is otherwise so 

 dense that the lymph could not pass through. 



The relations of the cells to the basement membrane are not always 

 the same. The simplest formation is with all the cells in a row and their 

 proximal ends resting squarely on the membrane. Or they may only 

 touch the membrane with one or more processes (see Fig. 192). Lastly, 

 some of them may grow up and away from the membrane altogether, 

 being supported by contact with those that remain in the original posi- 

 tion. This is true of the perceptory cells of the auditory or static epi- 

 thelium of vertebrates (see Fig. 192) as well as of some other forms. 



This last principle, when carried farther, results in many cells arising 

 from the row of the simple form of epithelium and lying in outer positions. 

 They remain attached to the 

 inner cells that are on or con- 

 nected with the " basement 

 membrane. Such is called a 

 stratified or multiple epithe- 

 lium, and is met with in the 

 integument of most vertebrates 

 and some invertebrate animals 

 (Fig. 49). 



In the skin of embryonic 

 mammals can be seen splen- 

 did series of developmental 

 stages of this form of cov- 

 ering. Here the epithelium 

 starts as a simple columnar 

 form, a plain row of cells 

 in section; in reality a single 



FIG. 49. Epithelium from the head of Sagitta hex- 

 aptera to show a simple stratified epithelium. 

 (From SCHNEIDER.) 



sheet of six-sided cells covering every part of the embryonic body. 

 On the umbilical cord this epidermis begins to change into a stratified 

 epithelium at a number of distinct points, one of which is shown in the 

 illustration (Fig. 50). It can be seen that the cells that form the outer 

 portions are not actually lifted out of the basal layer and pushed outward, 

 but that the cells of the basal layer divide by mitotic division, and that the 

 outer cells resulting from such divisions lie outside of the basal layer. 



