PLATE III. 

 Figure C. Magnification about 15 d. 



Lateral view of Embryo CLXIll; length, 9 mm.; age, 41/0 weeks. The 

 areas from which the skin has been removed are drawn from reconstruc- 

 tions, the remaining portions are drawn from excellent photographs. The 

 myotomes hide from view most of the deeper structures of the back and 

 body-wall. The superficial tissue of the myotomes has to a certain extent 

 fused, so that segmentation is becoming indistinct. In the region of the 

 arm, certain dense masses of tissue are represented in which later the mus- 

 culature of the arm is differentiated (see p. 23). In the region of the 

 forearm and hand this " premuscle " tissue has been removed so as to 

 disclose the dense mass of mesenchyme which at the centre of the limb-bud 

 represents the forerunner of the skeleton. The skeletal tissue is represented 

 with sharper outlines than in nature. Ulna, radius and hand plate are 

 shown. The musculo-spiral and median nerves may be seen reaching about 

 to the elbow. 



In the region of the leg the superficial tissue has been moved so as to 

 disclose the border vein, the sciatic artery, the skeletal rudiment of the 

 femur and hip-bone and the lumbo-sacral plexus. Into the formation of 

 the latter enter the five lumbar nerves and the first two sacral. 



