SECRETORY AM) EPITHELIAL CELLS 



43 



in mammals is composed of branched tubular o;lan(ls with a single 

 short excretory duct portion and two or more longer, slightly twisted, 

 tubular ])ortions composed of secretory cells. (Figs. 17 to 19.) 



Simple Alveolar (llaiuLs. —These glandular invaginations develop 

 a spherical termination instead of the tubular type just described. 

 In simple alveolar glands the enlarged si)herical terminal jiortion 

 is called an alveolus or acinus. These are connnon in the skin of 

 certain lower vertebrates but do not occur among the mammals. 

 An example of this type may be found in the skin of the frog where 

 a saccular base lies below the stratified squamous epithelium of 





n^ gH 



Fig. 17 



Fig. 



Fig. 19 



Fig. 17. — Simple tubular gland. 



Fig. 18.— Coiled tubular (sweat) gland. 



Fig. 19.— Simple branched tubular gland (stomach). 



the skin epidermis and connects with the surface by a passageway 

 through the strata of epithelial cells. This simple type of gland 

 may also be modified b\- having two or more alveoli attached to the 

 end of a single excretory duct, as in the case of the sebaceous glands 

 where alveoli are connected with the lateral walls of a common excre- 

 tory duct. (Figs. 20 and 21.) 



Compound Tuhiilar (Hands.— The formation of compound glands 

 may be visualized by assuming that, instead of stopping with the 

 simple tubular invagination of embryonic development, the early 

 invagination has gone on developing branches from its deeper 

 portions. These secondary branches give rise to tertiary ones, and 

 so on. (Fig. 22.) The outermost end-portions of the smallest 



