Jan. 20, 19 19 Variations arid Mode of Secretion of Milk Solids 97 



are certainly strong, whereas the proof for the cell-destruction hypo- 

 theses seems weak when analyzed in the light of the above facts, and, 

 in truth, in some particulars is contrary to known facts. The conclusion 

 that milk is a true secretion seems justified by what we know of the 

 mechanism behind such glands. 



The data above presented give us a criterion to judge the value of any 

 hypothesis for the origin of the milk solids from a common mother sub- 

 stance, such as has been suggested by Thierfelder (jj) and later by 

 Landwehr {17), to account for the derivation of casein and lactose from 

 nucleoproteids or glycoproteids of the gland cells by splitting. The 

 correlation of the solids-not-fat and fat might lead one to suppose such a 

 common origin for some component of such solids and the fat. This 

 can not be the case, however, as the correlation of fal: and of solids-not- 

 fat with amount of milk and age precludes that possibility, for if such a 

 common origin*occurred, the fat and solids-not-fat would necessarily be 

 correlated to these other variables by comparable amounts. The milk 

 components are not correlated equally with either milk quantity or with 

 age; consequently, the hypothesis of a common origin is not tenable. 



The correlations furnish the necessary evidence for the determination 

 of an important genetic relation between the hereditary factors for the 

 concentration of the butter fat and the solids-not-fat. Since the data 

 are taken from the presumably homogenous population of the advanced 

 registry Holstein-Friesian cows, linkage of factors for milk solids can 

 not be used to account for this correlation; for, granting the homo- 

 geneity of the population, the factors are as likely to be in opposite 

 chromosomes as in the same chromosomes — that is, calling one "a" and 

 the other "b," the parental chromosome content would be: 



ab a b 



or, reducing down, four chromosomes would be present in equal numbers 

 ab ^ a ^ b ^ These bred together would give the random dis- 

 tribution, considering that crossing over is as likely to occur one way as 

 another. This, however, is not what actually happens in our case. The 

 quantity of fat is correlated to the quantity of solids-not-fat per volume 

 of milk. This must mean that some of the factors responsible for fat 

 concentration are responsible for the concentration of sugar, protein, 

 or ash in cow's milk. 



