396 



ON INCREASE IN SIZE 



[PT. Ill 



in theoretical results, it is nevertheless based on sounder considerations 

 than the more ambitious 

 ones of other workers*. It 

 is probably legitimate to 

 assume that the laws of 

 growth before the formation 

 of the embryonic axis are 

 very different from what 

 they are afterwards. It is 

 also legitimate to assume 

 that the differences between 

 the velocity constants in the 

 three formulae are due to 

 the varying amount of or- 

 ganisation which has to go 

 on in each case before the ^ 

 formation of the primitive < 

 groove. Fig. 40 shows the o 

 straight-line relationships ? 

 found to hold by McDowell, ^ 

 Allen & McDowell in the 9 

 case of the guinea-pig, mouse ^ 

 and chick, and Fig. 41 

 gives further examples, from 

 which further variants of 

 McDowell's formulae could 

 easily be calculated. Clearly 

 in embryonic growth log. 

 weight is always propor- 

 tional to log. time. 



With respect to Fig. 39, 

 in which the weights of very 

 young chick embryos are 

 given, it should be noted 

 that the discrepancy would 

 naturally be expected to 

 occur only in the early 

 stages, for in the later ones the difference between conception age 

 and embryo age would be a smaller percentage of the total. The 



* But see p. 427. 



2 4 10 20 4060100 200 400 600 

 EMBRYO AGE IN }\J"^ OF DAY 



Fig. 40. 



