gA Journal of Agricultural Research voi. xvi. no. 3 



was 2.97 per cent butter fat and 8.87 per cent solids-not-fat, and the 

 evening milk was 4.31 per cent butter fat and 8.86 per cent solids-not- 

 fat. 



The significance of these facts as above established on the problem 

 of the mode of liberation of the constituents into cow's milk has been 

 overlooked. Before 1850 the prevailing opinion held that the milk 

 solids were filtered out by the mammary gland from the blood serum. 

 This view was shown to be incorrect by the fact that lactose is not present 

 in the blood and the fat percentage of the serum is not sufficient to 

 account for the fat in a single milking. To replace this old theory, 

 three major hypotheses have been put forth to account for the secretion 

 of the mammary gland : 



(i) Cells of the gland break loose bodily and disintegrate in the alveoli 

 to form the milk solids. 



(2) The portion of the cells toward the alveoli becomes loaded with 

 solids, breaks loose from the basal portion, and disintegrates to form the 

 milk solids. 



(3) The cells of the mammary gland secrete the materials of the milk 

 solids without themselves breaking down. 



In opposition to the first theory, it may be said that no such extensive 

 cell multiplication is witnessed in the mammary gland as would be 

 necessary to replace the cell destruction called for on the theory. This 

 disintegration, as pointed out by Heidenhain (ij) for the milk produced 

 by some cows in one day would require the replacing of all cells in the 

 udder at least five times a day, a replacement of cells unprecedented in 

 our knowledge of cell division. 



The second theory, suggested by Langer and ably supported by 

 Heidenhain (ij), Steinhaus (28), and Brouha (4), lays its foundation on 

 histological evidence. According to this evidence, the gland cells lengthen 

 out into the lumen of the alveoli. The projecting ends of these cells 

 become loaded with nutrients similar to milk solids. These projecting 

 ends disintegrate to allow the escape of these solids. The basal portions, 

 including a nucleus, are left to rebuild the cell and to enable it to repeat 

 the process. Steinhaus says that, in order to support this rebuilding, 

 mitotic divisions are frequent, and that the daughter nuclei which lie 

 on the outer portion of the cell often degenerate. 



The third theory lays its stress on analogy with the other secretory 

 glands without other supporting evidence than the negative evidence 

 of Bertkau (2), who says the disintegration appearing in the secretory 

 cells is due to imperfect fixation and that no necrobiosis of any kind 

 appeared. 



The above summary of the evidence for the three theories to account 

 for the introduction of the solids into the milk shows how contradictory 

 is the evidence so far presented. This contradiction, however, is not to be 

 wondered at The examination of the cells of the actively lactating 



