EXPERIMENTS ON THE FORE LIMB OF AMBLYSTOMA 419 
region of the fourth somite ventral and somewhat caudal to the 
pronephros.!° 
The limb bud on its first appearance is a nodule about one and 
a half somites in diameter, and is almost radially symmetrical. 
It soon acquires greater convexity on its dorso-posterior border 
(fig. 4), and may be said to ‘point’ in that direction, though 
the surface is rounded all over. From this period on growth is 
rapid. The direction of ‘pointing’ becomes more marked and 
the tip of the bud frees itself from the body wall, the axis of the 
limb making an angle of 380°-35° with that of the body when 
viewed from above, and pointing dorsally at about the same angle 
to the horizontal. The bud elongates into cylindrical form, being 
attached to the body wall obliquely at its base. During this proc- 
ess the axis of the limb is bent more nearly parallel to the median 
plane (figs. 5 and 6). As the bending proceeds the distal part 
of the limb becomes flattened in a plane about 45° to vertical, 
the dorsal border being nearest the body. A little later the first 
trace of the digitations appears at the extreme tip of the limb, 
which becomes more squarish and then slightly concave, the 
depression representing the notch between the first two digits 
(fig. 7). The digits elongate rapidly, as does the whole limb, 
but the joints are at this time not very distinct. While the notch 
between the fingers is still shallow, the dorsal border of the limb 
becomes distinctly convex, and at the same time the hand is so 
twisted as to le in a vertical instead of an oblique plane. The 
latter change is in reality partial pronation. The more lateral, 
which is morphologically the preaxial (radial) border, becomes 
ventral, the pollex lying on this side. The hand broadens out 
and the fore arm becomes somewhat flattened also. The elbow 
joint is now slightly flexed towards the ventral side (fig. 8). Up 
to this point the limb has no motility, the changes which have been 
described being due to growth and not to muscular action. 
10 The description of the normal development is given here merely for con- 
venience, and only those phases which concern the experiments are considered. 
The subject is fully treated in papers by Gétte (’79), Strasser (’79), Rabl (’01), 
with especial emphasis on external form and skeleton. Miss Byrnes (’98 a) has 
described the relation of the limb bud to the body wall and myotomes in the 
early stages. 
