14 Development of the Limbs, Body-wall and Back in Man 



thigh), from the distal portion are developed forelimb (forearm and leg) 

 and extremity (hand and foot). The place where the distal part of the 

 limb joins the base represents the future elbow or knee. These two 

 joints are formed in a flexed position (see especially Figs. F, G, H and I, 

 Plates VI-IX). 



The constriction mentioned above serves to designate the differentia- 

 tion of the distal part of the limb-bud into the forelimb and extremity. 

 Opposite the constriction the forelimb is formed; immediately distal to 

 it the extremity (Figs. 8, 9, 10, 11, 12, 13, 14, 15). 



The hand and foot are at first flattened, disk-shaped bodies. Digita- 

 tion is first marked on the lateral surface (see the hand in Fig. 9), and 

 then in the free margin also (Fig. 10). 



Differentiation of the base of the limb-bud is somewhat less easy to 

 follow than that of the distal part. This is due mainly to the fact that 

 many distinguishing structures are at first deep-seated. As the mus- 

 culature, however, becomes distinctly differentiated, the various main 

 groups of muscles give rise to distinctive external characteristics. 



The limbs at the stage shown in Fig. 15, p. 9, and in Figs. F, Gr, 

 H and I, Plates VI-IX, exhibit rudiments of most of the structures 

 characteristic of the adult limbs. Much growth and shifting of parts, 

 however, is necessary before adult conditions are reached. 



The great curvature of the axis of the body opposite the limbs 

 during their formation has been noticed by His." 



The arm-bud at first lies opposite the 5th to 8th cervical and 1st 

 thoracic myotomes. As it grows in size the area of attachment to the 

 body enlarges and extends on its cephalic side to the level of the 3rd 

 cervical neural process (see Embryo CIX, Fig. 10). We find also that the 

 shoulder comes to lie closer and closer to the precervical sinus until the 

 embryo reaches a length of about 20 mm. As development still further 

 proceeds the shoulder gradually migrates caudally and the distance be- 

 tween the shoulder and precervical sinus increases imtil the adult position 

 is attained. Simultaneous with this caudal migration of the arm, the 

 lower portion of the ventral wall of the neck appears. 



The posterior limb arises slightly later than the anterior. Through- 

 out its development the leg lingers somewhat behind the arm. The 

 base of the leg-bud is at first opposite the 1st to 5th lumbar and the 1st 

 sacral myotomes. Gradually the base extends so as to include the 

 region opposite the second and third sacral myotomes and the upper 



9 Zur Geschichte des Gehirns etc., Bd. XIV, der Abhandl. der Mathematisch-pbysi- 

 clien Classe der Konigl. Sachs. Gesellschaft der Wissenschaften, p. 381. 



