394 ON INCREASE IN SIZE [pt. iii 



very different from what afterwards obtains during embryonic develop- 

 ment. There is, therefore, much justification for the view of McDowell 

 and his collaborators. Brody himself had come nearly to this position 

 without recognising it, for, in a paper published in 1923, he had 

 pointed out that, during a short period in the early stages of growth 

 (or regeneration) the apparent observed speed seems to be slower than 

 would be expected. Thus the curve of the fitted equation cuts the 

 time axis not at zero, the beginning of growth, but a little later. He 

 advanced the explanation that Durbin had already applied to the 

 initial slow phase in the regeneration of tadpole tails, namely, that a 

 "cap of embryonic cells" was first formed, following in its growth 

 quite different laws from the subsequent process as a whole. "It is 

 suggested", said Brody, "that the apparent initial slow phase of 

 growth of the individual from the fertilised egg is due to a similar 

 qualitative growth." 



(Estimated weights of eggs are shown in Table 53.) 

 McDowell and his collaborators proceeded to show that a similar 

 formula would fit very well the curves of growth for the guinea-pig 

 (Draper; Hensen; Ibsen & Ibsen), the mouse (McDowell, Allen & 

 McDowell) and the chick (Murray; Needham; and Schmalhausen). 

 For the mouse it was : 



log W = 3-649 log {10 (^ - 7-2)} + 8-6587; 



and for the guinea-pig it was : 



log W = 3-987 log {10 {t - 12)} + 5-1839; 



The significant thing about these empirical formulae is the deduction 

 of a certain time in each case from the conception age, thus 7-2 days 

 in the case of the mouse, 1 2 days in the case of the guinea-pig, and 

 0*5 day in the case of the chick (Allen & McDowell). The evidence 

 on which McDowell and his collaborators rested their case for this 

 shortening of the development time was drawn from various sources ; 

 thus, from their own histological observations they found that the 

 primitive streak in the mouse embryo appeared about the 7th day 

 of development, for 6-day embryos show no mesoderm, while 7-day 

 ones do, and usually the primitive groove as well. Sobotta's work is 

 in agreement with this estimate. Their estimate for 12 days as the 

 time taken for the embryo guinea-pig to reach the primitive-groove 

 stage was based on the generally accepted work of BischoflT and 

 Lieberkiihn, while, for the chick, Duval, whose illustrations are the 



