60 



ZOOLOGY. 



is in special need of 

 a continual supply of 

 plasma, and hence the 

 two tissues secretory 

 epithelium and vascular 

 connective tissue are in 

 a close physiological con- 

 nexion. It is often 

 needful that the liquid 

 secreted by a given area 



Fig. 19. STRUCTURE OF GLANDS. 



1, simple pit ; 2, flask-shaped gland with short 

 duct ; 8, 4, more complex compact glands. 

 Vascular tissue dotted. (After Huxley.) 



of epithelium should be con- 

 centrated in a smaller area : 

 this is effected by the double 

 tissue being thrown into folds 

 of some kind. It is by this 

 process of folding and branch- 

 ing that the complex glands 

 of the body are 

 produced. Fig. 19 

 gives a diagram- 

 matic representa- 

 tion of the suc- 

 cessive stages of 

 complication. 



As a gland be- 

 comes more and 

 more complexly 

 branched, the 

 tubular portion near the open- 

 ing may become drawn out 

 into a duel of some length, 

 whose epithelium is no longer 

 secretory. 



The sweat-glands (fig. 13) 



Fig. 21. 



TRANSVERSE 



SECTION or 



SIMPLE 



TUBULAR 



GLAND. 



Fig. 20. SIMPLE GLANDS IN WALL 

 OF STOMACH. 



(After Klein.) 



