CELLULAR DIFFERENTIATION 51 



69. Bounding Epithelium. The ordinary protective epithelium may be made 

 up of cells cuboidal in shape (Fig. 16, B), or columnar (Fig. 16, A), or much 

 flattened (Fig. 16, Q. In extreme cases of flattening and hardening we have 

 squamous epithelium, e.g., the outer cells of the human epidermis. Motile proto- 



FIG. 19. Sensory and muscular epithelium. A, sensory epithelium, from Worm, showing 

 some of the epithelial cells (e) modified into sensory cells (s). B, epithelial cells from Hydra showing 

 contractile or muscular processes at base (m). 



Questions on the figures. Is there anything to suggest that the sensory cells 

 are modified epithelial cells? What are the principal changes which they have 

 undergone as compared with the unmodified epithelium? 



FIG. 20. 



FIG. 20. Diagram of a portion of the ovary of Sea-urchin showing the eggs arising from the 

 epithelium (reproductive epithelium) by constriction, e, epithelium; <?, ova in different stages of 

 growth. 



Questions on the figure. What is an ovary in its simplest form? Is the re- 

 productive epithelium ectoderm al, entodermal, or mesodermal in origin, as a 

 rule? 



plasmic projections often extend from the free surface of the epithelium. Flagel- 

 late epithelium (Fig. 79, D) has one such projection from each cell, whereas ciliate 

 epithelium (Fig. 16 D, E) has numerous small ones. Cilia are more common in 



