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Special Vertebrate Organogenesis 



it ventrally. After this period the prominence 

 on the side of the embryo becomes distinct 

 (Fig. 152C, stage 33), but it is not until 

 several days later that the extremity itself 

 appears on the surface as a sharper elevation 

 in the region of the fourth somite. 



The limb bud at first is a nodule about 

 one and a half somites in diameter, and is 



being nearest the body. A little later, the 

 first trace of the digitations appears at the 

 extreme tip of the limb, the depression rep- 

 resenting the notch between the first two 

 digits (Fig. 152G). The digits elongate 

 rapidly, as does the whole limb, but the 

 joints are at this time not distinct. The 

 dorsal border of the limb becomes distinctly 



Fig. 152. 



almost radially symmetrical. It soon ac- 

 quires greater convexity on its dorsoposterior 

 border (Fig. 152D), and may be said to point 

 in that direction, though the surface is 

 rounded. From this period growth is rapid. 

 The tip of the bud frees itself from the body 

 wall, the axis of the limb making an angle 

 of 30 to 35 degrees with that of the body 

 when viewed from above, and pointing dor- 

 sally at about the same angle to the hori- 

 zontal. The bud elongates into cylindrical 

 form, being attached to the body wall 

 obliquely at its base. During this process the 

 axis of the limb is more nearly parallel to the 

 median plane (Figs. 152£ and F). The distal 

 part of the limb becomes flattened in a plane 

 about 45 degrees to vertical, the dorsal border 



convex, and at the same time the hand is so 

 twisted as to lie 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 and the 

 forearm becomes somewhat flattened also. 

 The elbow joint is now slightly flexed to- 

 wards the ventral side (Fig. 152//). The 

 limb is not motile, the changes being due to 

 growth and not to muscular action. The 

 third and fourth digits appear successively 

 on the ulnar (dorsal) border of the hand, 

 first as nodules which slowly elongate, the 

 fourth being considerably behind the third 

 in its development (Fig. 1527). The arm 



