392 



ON INCREASE IN SIZE 



[pT. m 



and Schmalhausen made weighings of embryos between the second 

 and the sixth day of incubation, it was found that Murray's formula 

 did not hold for these earlier stages. Fig. 39 taken from the paper 

 of McDowell, Allen & McDowell, illustrates this point. The broken 

 line is drawn to Murray's formula, and the dotted line is an extra- 

 polation of his curve which I made on the assumption that embryos 

 grew at the same rate before 5 days as between 5 and 7 days, i.e. 

 exponentially. The circles with dotted centres are the values experi- 

 mentally obtained by me, the dots are those obtained by Schmal- 

 hausen, and the cross within a circle is Murray's earliest figure. 



4.6 



4.2 



>.? 



3 J.4 



Of 



3 



2.6 



2.2 



0.65 0.70 0.75 



0.85 0.90 . 0.95 1.00 1J)5 



L05 incubation a^e (dsajs) 

 Fig. 38. 



U5 



1.20 



1.25 UO 



Murray's formula gives a line consistently above the experimental 

 points for this early period, and the exponential extrapolation is quite 

 at variance with them. But McDowell, Allen & McDowell evolved 

 an equation which fitted these early points (the solid line) as well 

 as all the later ones, as follows : 



log W = 3-436 log {10 (r- 0-5)} + 7-626. 



This new equation was based on an entirely different viewpoint 

 from that of previous workers. McDowell and his collaborators 

 regarded not "conception age" but "embryo age" as the right zero 

 hour to take in growth calculations. It had always been assumed 

 previously that conception or even insemination was the right 

 starting-point, and Brody & Ragsdale and Brody based their method 

 for finding age-equivalence in animals on this view, while Friedenthal 



