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



present (3o, 8c, 12t, 51, 5s, 5-6c). The extensions of the myotomes 

 within the body Avail are pictured. The base of the arm-bud lies oppo- 

 site the 5th to 8th cervical and 1st thoracic myotomes; that of the leg- 

 bud opposite the 1st to 5th lumbar and 1st sacral myotomes. The arm- 

 bud projects caudally, the leg-bud outwards and slightly caudally. 



Embryo A. 



In Fig. 7 is represented the His embryo A; length, 7.5 mm.; age, 

 about 4 weeks. Thirty-five myotomes are pictured (8c, 12t, 51, 5s, 5c). 

 The arm-bud lies opposite the 5th to 8th cervical and 1st thoracic; the 

 leg-bud lies opposite the 1st to 5th lumbar and 1st sacral myotomes.^ 

 Both project caudally. Both show a slight division into segments. 

 This, however, is much more marked in the following embryo. 



Fig. 7. 

 X about 5 d. 



Embryo CLXIII. 



In Fig. 8 is represented Embryo CLXIII; length, 9 mm.; age, about 

 4|- weeks. Two photographs of this embryo are shown on Plate I. 

 Thirty-three myotomes are present (8c, 12t, 51, 5s, 3c). The base of 

 the arm lies opposite the 4th to 8th cervical and 1st thoracic, and that of 

 the leg opposite the 1st to 5th lumbar and 1st to 2nd sacral myotomes. 

 The arm projects nearly caudally. A constriction on the cephalic and 

 caudal borders separates the rounded upper arm from the flattened 

 lower arm and hand. The constriction on the caudal border is close 

 to where the arm joins the body-wall, while that on the cephalic border 

 is at a point some distance from the body-wall. This difference on the 

 two borders is to be correlated with the caudal projection of the arm. 



5 This statement is based on the drawing given in Fig. 3, Plate I* of the Atlas. 



