II THE METAZOA 63 



This mode of development is, however, not entirely with- 

 out parallel among the Protozoa. In the colonial Volvox 

 (p. 43, Fig. 17) it will be remembered that male cells or 

 microgametes (sperms) and female cells or megagametes 

 (ova) are developed, and that by the coalescence of a 

 microgamete with a megagamete a compound cell, the 

 zygote (oosperm), is formed, which undergoes division to 

 give rise to an adult Volvox. 



As the various parts become gradually moulded from the 

 cells of the germinal layers, the form and arrangement of 

 the cells of the different parts become modified in different 

 ways, so that the cellular structure comes to differ widely; 

 and, as a result, we find in the fully formed animal a 

 variety of different kinds of material, — tissues, as they are 

 termed, — such as muscle, bone, gristle, nerve, all derived 

 from the cells of the germinal layers. Of such tissues the 

 following are the most important. An epithelium is a thin 

 stratum of cells covering some surface, external or internal ; 

 it may be one cell thick, or several cells thick. The cells of 

 which an epithelium is composed vary greatly in form in 

 different cases (Fig. 28) : they may be beset at their free 

 surfaces with cilia (#), like the cilia of the Infusoria, or with 

 flagella, like those of the Mastigophora (/), or may be 

 amceboid (k), sending out pseudopodia like a Rhizopod. 

 The epithelium which covers the outer surface is known 

 as the epidermis or deric epithelium ; that which lines the 

 interior of the digestive organs is the enteric epithelium. 



Glands (Fig. 29) are formed by modification of epithe- 

 lial cells. In many cases a single cell of the epithelium 

 forms a gland, which is then termed a unicellular gland 

 (A, B). The secretion (or substance which it is the func- 

 tion of the gland to form or collect) gathers in such a case 

 in the interior of the cell, and reaches the surface of the 



