NOURISHING MEMBRANES 



499 



the mammary glands from 

 which the young suck an 

 epithelial, glandular se- 

 cretion called the milk. 

 These glands consist of 

 an embryonic invagina- 

 tion of the stratified epi- 

 thelium into a series of 

 aveolo-tubular glands 

 (Fig. 468). In the acini 

 and ducts of these invagi- 

 nations, only the basal 

 layer of cells persists as 

 a simple epithelium which 

 is the secreting layer of 

 the gland. 



The mode of secretion 

 is peculiar, when we con- 

 sider the fact that other 

 oils (sebaceous and odorous) are usually produced by a degeneration 



FIG. 468. An acinis from a functional mammary gland of 

 the cat. The lumen is filled with the watery secretion. 

 Several of the cells are secreting large fat droplets which 

 are stained black with the osmic acid. X 1300. 





FIG. 469. Part of the foetal membranes of a tern, Larix. ent., entoderm whose cells are engaged 

 in securing nourishment from the yolk. X 435. 



and disquamination of the cell. In this form of tissue the secretion is 

 formed in the distal portion of the rather short cells, and is carried to 



