4 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
pregnant cow contains certain substances called “milk- 
forming substances.” ‘During pregnancy the greater 
portion of these substances is required for the nourish- 
ment and development of the foetus, but sufficient is 
left over to stimulate the development of the glandular 
tissue in the udder. When the fcetus is developed, all 
are available for action upon the udder and secretion is 
stimulated. 
At the time of parturition the udder does not secrete 
milk, but a substance called colostrum. The alveoli at 
this period contain many cells, entire and disintegrated, 
and leucocytes are also numerous in the interalveolar 
connective tissue. The secretion is therefore rich in cells. 
It also contains comparatively large, round bodies which 
have the appearance of masses of fat grobules. These 
are the so-called colostrum corpuscles, which are re- 
garded by some as leucocytes which have taken up a large 
number of fat globules, and by others as exfoliated epi- 
thelial cells containing masses of fat globules. A cow 
in this stage of lactation is said to be “fresh.” 
During the first week the secretion gradually changes 
to milk. The alveoli of the udder are not all active at 
this time, but those which are inactive and which have 
not undergone permanent involution resume their func- 
tion within the succeeding two or three weeks, when the 
secretion of milk reaches its highest point. Usually by 
the end of the first week the leucocytes have disappeared 
from the alveoli and interalveolar tissue and very few 
cells are present in the milk. But if milking is delayed 
or is incomplete at any time, or if stasis of milk occurs 
from any cause, leucocytes again invade the alveoli and 
interalveolar tissue in large numbers and become nume- 
rous in the milk. Their appearance under these condi- 
