SEGREGATION OP PAT FACTORS IN MILK PRODUCTION. 
BY F. B. HILLS AND E. N. BOLAND. 
The need of an investigation into the problem of inheritance of fat 
in milk, has long been recognized by the practical milk producer and 
the physiologist. For the former, a solution of the problem would sim- 
plify, very markedly, his breeding operations, as well as increase the cer- 
tainty of his results. For the latter, a knowledge of the fat producing 
possibilities of the animal genetically would give an index to the physi- 
ological limits of fat formation, and its relation to metabolism. 
The title of this paper might indicate that the problem has been solved, 
but in this sense, the title is a misnomer, for the paper can merely throw 
a little light, perhaps, on some of the work to be done. It records the 
principal discoveries made in the pursuance of a Master ’s degree thesis 
in the breeding laboratory of the Animal Husbandry Department of this 
college. 
With the exception of the data collected from the microscopical exami- 
nation of many samples of milk in the laboratory, the source of the data 
studied is the Advanced Registry Official records of the Holstein-Friesian 
Association. The number of animals listed is large and the field little 
exploited, although worthy of systematic consideration, since the records 
have been kept for a sufficient length of time to include the performance 
of many generations. 
The commonly accepted theory of milk secretion, is that first proposed 
by Danger and since slightly modified by Steinhaus, Brouha and others. 
Dr. Marshall, in his “ Physiology of Reproduction,” has outlined this 
theory somewhat as follows : Some of the cells of the gland lengthen out 
so that their ends project freely into the lumina of the alveoli, and prob- 
ably undergo cell division. The projecting portions then disintegrate, 
before or after becoming detached, and the cell substance passes into solu- 
tion to form the albuminous and carbohydrate constituents of the milk. 
The fat droplets, which collect in the disintegrating part of the cell, give 
rise to the milk fat. The basal portions of the cells remain in position, 
without being detached, and subsequently develop fresh processes, which 
