SECT. 2] 



AND WEIGHT 



393 



had shown a similarity in relative growth-rates by plotting the log. 

 weights against log. conception ages. McDowell and his colla- 

 borators, on the other hand, suggested that the time of growth ought 

 rather to be calculated from the time at which the embryo first 

 begins to have an axis, i.e. from the primitive-streak stage. Thus the 

 major differences between the pre-natal growth of the guinea-pig, 



WEIGHT 

 . INMGS. 



■200 

 -180 

 •160 

 •140 

 -120 

 -100 

 • 80 



■ 60 



■ 40 

 - 20 



DATA OF 

 • SCHMALHAUSEN 

 ONEEDHAM 

 e MURRAY 



INCUBATION 1 

 AGE 



FREQUENCIES 



©85 077 ©88 © ©83 



77 ffi200 



Fig. 39- 



the mouse, and the chick would be accounted for by the varying 

 times taken to get through the preliminary work of arrangement and 

 organisation. Processes such as gastrulation, according to this view, 

 would involve a law of growth so different from the later axial type 

 that no formula should be expected to cover the two. We have already 

 seen in the case of the rat's egg that some considerable time may 

 elapse between the time of fertilisation and that of fixation to the 

 uterine wall, during which the supply of nutrient material may be 



