Mathilde L. Koch 277 



It remains therefore to inquire whether the chemical and behav- 

 ior relations between the rat and pig which have just been pointed 

 out, occur at equivalent ages in these two forms. 



Great difficulty was experienced in finding any statements con- 

 cerning the age of the pig fetuses. The statements of different 

 authors did not always agree, but the two which agreed closest 

 were those of Bradley 17 and Coe. 18 Bradley compared the length 

 of the embryos with the time from coition; Coe estimated the age 

 from the rate of development of embryos of other mammals. 

 While considerable uncertainty thus attaches itself to these figures 19 

 it may be assumed that the 50 mm. pig fetus is about 40 days old 

 from conception: the 100 mm. fetus is 55-62 days; and the 200 mm. 

 fetus is from 88-90 days from conception. 



We find in the rat the period of gestation is 21 days and its span 

 of life three years (Donaldson), or a total age of 1116 days; in 

 the pig the period of gestation is 125 days and its normal span of 

 life, as far as could be ascertained, is 20 years, 20 or 7425 days. The 

 rat, therefore, lives about one-sixth as long as the pig. Assuming 

 that the rat at birth has lived TTTG", or ^V, of its total life, the 

 60-day pig fetus will have lived yfio, or TTS, of its life. It appears 

 then, if the total length of life given is correct for both animals 

 and the numbers used for the divisors in each case are really com- 

 parable as they stand, that we do not have corresponding physio- 

 logical conditions of the brain at equivalent ages, for these brains 



17 O. C. Bradley: On the Development of the Hind Brain of the Pig, 

 Journ. of Anat. and PhysioL, xl, Part I, p. 1. 



18 Mendel refers to Professor Coe in Chemical Studies on Growth. I. 

 The Inverting Enzymes of the Alimentary Tract, especially in the Embryo, 

 Amer. Journ. of PhysioL, xx, p. 90, 1907-1908. 



19 Bradley makes the statement, that "although the age of the different 

 embryos is given, it is not intended that it should signify more than the 

 time which elapsed between the time of coition and the time when the mother 

 was destroyed In embryos taken from two litters it not infre- 

 quently happens that those which should be further advanced in develop- 

 ment, judging from the period which has elapsed since sexual congress took 

 place, are as backward, or even more backward, than those of the 'younger* 

 litter." A more definite way to determine the age of an embryo would be, 

 according to Mall, by ossification. No data were available however for a 

 comparison between the rat and the pig. 



20 Longevity, Encyclopaedia Britannica (Eleventh Edition), xvi, p. 975. 



