418 ROSS G. HARRISON 
plates may be observed through the skin. ‘Though there is no 
distinct limb bud present, the region centering in the fourth seg- 
ment just ventral to the pronephros contains the material that 
will give rise to the limb. A little later, in the standard operat- 
ing stage when the tail bud is more marked (fig. 2, stage 29), 
sections show that the somatopleure ventral to the pronephros 
is thickened. Still we can scarcely speak of a ‘limb bud’ on the 
Figs. 1 to 3 Embryos of Amblystoma punctatum in the stages used for op- 
eration. X10. A, D, P, V, mark the four cardinal points (anterior, dorsal, 
posterior, ventral) of the limb bud (LB); MY3, third myotome; PN, pro- 
nephros. Figure 1. Earliest stage (stage 25). Figure 2. Standard operating 
stage (stage 29). Figure 3. Oldest stage (stage 33). 
surface of the embryo, for the pronephric tubules are the chief 
eause of the swelling in that region, the somatopleural thickening 
merely serving to round it out on the ventral side. After this 
period the somatopleural proliferation increases, making the 
prominence on the side of the embryo more distinct (fig. 3, stage 
33), but it is not until several days later that the extremity 
itself appears on the surface as a more sharp elevation in the 
