THE RABBIT. 59 



lost in the adult rabbit, but well developed at an early 

 stage in the embryo. Here the bladder of protoplasm of 

 each cell encloses as much of a watery liquid as it can hold. 

 We all know how firm a bladder feels when fully distended 

 with air or water this firmness is not a property of the 

 material of the bladder, for that is soft and flexible when 

 the fluid inside has escaped. Exactly the same is the case 

 with a green stem or leaf of a plant. When dry it is limp 

 and droops, though normally it is firm and elastic. In the 

 same way notochordal tissue, consisting entirely of cells, 

 each of which is a bladder full of liquid, is firm and elastic, 

 and has much the same consistency as cartilage. As we 

 shall see later on, notochordal tissue precedes cartilage as a 

 firm basis for certain muscles to be attached to, much in the 

 same way that cartilage precedes bone, 



16. Tissue-Complexes. The two remaining groups of 

 tissues muscular and nervous we shall deal with in 

 future chapters. Before ending this one we must say a 

 word on tissue-complexes. 



Very few of the simple tissues we have been describing 

 form actual organs by themselves alone. We saw ( 6) 

 that epithelium contains no blood-capillaries or lymphatics, 

 but receives its nutrition by the exudation of plasma 

 from the capillaries of the subjacent layer of connective 

 tissue. The only exceptions are perhaps some endothelia, 

 especially those of capillaries. The connexion between 

 an epithelium and its underlying connective tissue is a 

 very close one, and often they form a single anatomical 

 structure, separable into two parts only by microscopical 

 examination. Thus the epidermis and the underlying 

 connective tissue, the dermis, together form the skin ; the 

 lining epithelium of the alimentary canal and its connective 

 tissue form the so-called " mucous membrane " ; the en- 

 dothelium of the ccelom and the associated fibrous con- 

 nective tissue form the peritoneum, mesentery, pleura and 

 pericardium. 



17. Glands. Still more is this the case when we 

 come to the (/lands : here the epithelium is secretory, and so 



