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



The mode of Secretion the cat - The lumen is filled with the watery secretion. 

 , . , Several of the cells are secreting large fat droplets which 



IS peculiar, when we COn- are stained black with the osmic acid, x 1300. 



sider the fact that other 



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



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 



