8 Milk and Its Products 
part of the albumin is in some way changed dur- 
ing its passage from the capillaries, and appears in 
the cavity of the follicle as the casein of the milk. 
fig. 1. Section through alveoli of the mammary gland of the dog in first and 
second stages of secretion. From Meade Smith, after Heidenhain. 
When milk is being secreted, the lining cells of 
the follicle are in a state of constant activity. 
New cells are constantly being formed by budding 
or: fission (the cell elongates, a partition forms 
across it, and the two halves so divided enlarge to 
the size of the mother cell, and there are two cells 
where but one existed before), and older cells are 
as constantly breaking down. While this is going 
on, the cell contents, consisting mainly of protoplasm, 
become changed into a globule of fat, and the 
globules so formed are either extruded through the 
cell wall into the cavity of the follicle, or set free 
by the breaking down and reabsorption of the cell 
wall. In all probability both processes take place. 
Small portions of the fat may also be carried over 
directly from the blood and appear in the milk 
without change. The milk sugar is probably formed 
