50 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



At these early ages, it seems justifiable to treat these data as homo- 

 geneous, in spite of the fact that some of the fetuses were largely of 

 Holstein blood, while others were largely of Guernsey blood. In the 

 same series may be included Eckles' analysis of a full-term Holstein 

 fetus, weighing 95 pounds. With its placenta tbis fetus contained 9162 

 gms. of protein. Since the average weight of Holstein calves at birth is 

 89 pounds, instead of 95 pounds, the protein content of the full-term 

 Holstein fetus with its membranes may be roughly estimated at 



89 



-— X 91(52 = 8585 orams. 

 95 



A mathematical analysis of the data. — The data at hand evidently 

 give only a fragmentary picture of the intra-uterine growth in protein 

 of the bovine fetus. A mathematical description of the data by a con- 

 tinuous function is essential to any effective use of them. The growth 

 in weight of the chick embryo has been closely described by Murray (^*) 

 by means of the simple exponential (or parabolic) equation. 



W = kt" (25) 



in wliich W is the weight of embryo, and t is the incubation age. In 

 adapting this equation to the intra-uterine growth of the mouse embryo, 

 MacDowell and Allen(^^) found it necessary to change the significance 

 of t from conception age to something less, specifically 7 days less. This 

 reduced age they call the " embryo age " on the following basis : 



" Since the first stages of development of a mammal consist of the 

 formation of the pro-embryo, a consideral)le period elapses before the 

 first organization of the embryo proper. This is the justification for 

 assuming embryo age to be less than conception age. In the mouse the 

 first differentiation of the embryo proper (primitive streak) is not found 

 before the end of the first week. Thus the embryological evidence bears 

 out the purely graphical resiilt obtained by shifting the age (t — 7) until 

 the embryo weights fit a logarithmic straight line." 



Thus, the mouse embryo at seven days after conception and the chick 

 embryo at the beginning of incubation are in practically the same stage 

 of development. Evidently the shifting of the embryo age seven days 

 ahead of the conception age will also include whatever error is made in 

 measuring conception age from the time of copulation with the male. 



In accordance with the experience of MacDowell and Allen, the nitro- 

 gen-growth data of bovine fetuses were not very closely described by an 

 equation of type (25), but if t is taken as one month less than the age 

 counted from the date of breeding, a fairly satisfactory fit is obtained, 

 the equation becoming 



W = 0.143 (t-1)*-"' (26) 



