SECT. 2] 



AND WEIGHT 



443 



stomach until the end of the embryonic period. It is certainly in- 

 teresting that organs so different in origin and nature as the eye and 

 the mesonephros should be similarly affected by spurts of growth 

 at various stages, and Schmalhausen concluded that this was an 

 argument against the hypothesis of specific organ-stimulating sub- 

 stances, the presence of which would from time to time cause more 



1 Z 3 V 5 6 7 a 3 10'^ 1Z ^3 1'^ -75 16 17 13 13 20 Zi 

 Days 



Fig. 57. C = brain; £; = hind limb; A" = whole body; Z-t = liver; Li = lens; Af=mesone- 

 phros; Oc = eye; P = lung; Af /n = metanephros ; 5^ = stomach; G= gonads. 



rapid growth in one place of the embryo than in another. It looks 

 much more as if growth-promoting substances were passed round 

 in the embryonic circulation at certain definite intervals, and so 

 exercised an effect on a large number of different organs. In this 

 connection, the recent work on the growth-promoting substances of 

 egg-yolk should be borne in mind, and the experimentally deter- 

 mined cycles of varying permeability to fat-soluble and water- 

 soluble substances in the walls of the vitelline blood-vessels. One 

 relation which seems clear from Schmalhausen's work is that growth 



