104 DEVELOPMENT OF EPITHELIUM. 



on repeating itself over and over again in the club-shaped ends 

 of the processes. The intermediate apparatus of nutrition takes 

 no part whatever in the process ; nay, it is interesting to notice 

 how it seems as it were to melt away before the advancing pro- 

 trusions, contenting itself with the function of a mere stop-gap, 

 the only function left open to it : it furnishes the interstitial con- 

 nective tissue, the blood-vessels and the lymphatics, which, as we 

 know, accommodate themselves humbly to the formal arrange- 

 ment of the glandular tubuli, acini, &c. 



§ 83. Matters take a very different course in the skin and 

 mucous membranes. Their growth is regulated not by the epithe- 

 lium, but by the conjoint blood-vascular and connective-tissue 

 system, whose morphological elements (papillae, membranes, &c.) 

 determine the form of the surface, upon which the epithelium 

 exists only as a tegumentary investment. This must be borne in 

 mind when we find (in the course of the ensuing paragraphs) 

 that the matrix of the surface-epithelium is situated, not in its 

 own substance, but in the underlying connective tissue. 



The cells of every epithelial stratum of considerable thickness 

 exhibit certain characteristic variations in form, which may be 

 viewed broadly as differences due to age — as phases of develop- 

 ment. The youngest elements, remarkable for their small size, 

 their softness and want of cell-membrane, are most deeply placed ; 

 Pjq 24 t^^^y ^^0 01^ ^^^^ upper limit of the con- 



nective tissue. As we proceed out- 

 wards, the cells increase progressively 

 in size, their membranous investment 

 grows more and more distinct, they 

 The epithelium of the urinary exhibit a more or less characteristic 

 bladder in section. ^^^^^ rj^j^^.^, ^^^^ -^ connected partly 



with variations in their function (cylindrical epithelium) ; it is 

 partly due to a conflict between the forces which tend to make 

 the cell grow uniformly in all directions, and the limitations of 

 space which permit its growth to take place in certain directions 

 only. A pregnant illustration of the results of this conflict is 

 afforded by a vertical section through the epithelial lining of the 

 urinary bladder (fig. 34). '\Yq can clearly distinguish three 

 layers, each of which is made up of cells of a particular sort, 

 differing characteristically from the other two. In immediate 

 contact with the connective tissue is a single layer of small 



