MORPHOLOGY OF HIGHER TYPES 



111 



instances the secreting cells may be grouped instead of isolated, in which 

 case they form what may be called a surface gland, as in C. A greater 

 secreting surface is secured when a group of secretory cells drops below 

 the general surface as in D. This constitutes a simple gland, that is, 

 one which consists of a single depression and is not branched. It is pro- 

 duced by a process of invagination or infolding and may be tubular 

 (Fig. 73, F) or alveolar (D and E) depending on whether the channels are 

 of uniform bore or are inflated. If the invagination is carried far below 

 the general surface, a neck (E) may be produced through which the 



Fig. 73. — Types of secreting surfaces and glands. A, scattered gland cells (two goblet 

 cells containing secretion in the darkly stippled goblets) ; B, gland cell enlarged and dropped 

 below general level; C, group of secreting cells dropped slightly below the general level; 

 D, a simple multicellular gland; E, alveolar gland with neck; F, tubular gland; G, com- 

 pound alveolar gland; H, compound tubular gland; I, lumen; m, mouth; n, neck; v, acini. 

 Secreting portions of the glands are stippled. 



secretions are conducted to the surface. The neck is usually composed 

 of non-secreting cells. Glands of this sort are common in the skin of 

 amphibians. 



From a simple gland, as shown in Fig. 73, E or F, a co7upound gland 

 may be formed, as in G or H , by secondary invaginations from the 

 original invagination. Compound glands are common in the wall of 

 the esophagus of mammals and certain amphibians. In the proventricu- 

 lus (first division of stomach) of the chicken or pigeon are large com- 

 pound glands. 



