CELLULAR DIFFERENTIATION. 



45 



connection with what will be described presently as the nervous tissues of 

 the central nervous system. The sensory cells are typically thread-like 



FIG. 17. 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 (n). 



Questions on the figure. Is there anything to suggest that the sen- 

 sory cells are modified epithelial cells? What are the principal changes 

 which they have undergone as compared with the unmodified epithelium? 



FIG. 18. 



FIG. 1 8. Diagram of a portion of the ovary of Sea-urchin showing the eggs arising 

 from the epithelium (reproductive epithelium) by constriction, e, epithelium; o, ova 

 in different stages of growth. 



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

 the reproductive epithelium ectodermal, entodermal, or mesodermal in 

 origin, as a rule? 



or hair-like in form, often extended as fine fibres at the inner end, whereby 

 connection is established with the nerves (Fig. 17, A). 



