PLATE II. 

 Figure A. Magnification aboiit 20 d. 



Drawing from a reconstruction of the region of the posterior extremity 

 in Embryo II; length, 7 mm.; age, 26 days. The xvii (9th thoracic) to the 

 xxix (4th sacral) spinal segments, and, at the left, the right leg and a 

 portion of the body-wall are represented. From the region ventral to the 

 spinal-cord unformed mesenchyme, the aorta and left cardinal vein, and 

 the smaller blood-vessels, the intestines and urino-g'enital organs have been 

 removed. From the spinal segments the myotomes of the left side have been 

 removed with the exception of the last two thoracic, the first two lumbar 

 and the fourth sacral. Half encircling the spinal-cord the scleromeres may 

 be seen at the distal third of each spinal segment. The chorda dorsalis 

 runs ventral to the midline of the spinal-cord. The 9th to the 12th right 

 thoracic mj^otomes and nerves of the right side may be seen extending 

 for a short distance behind the lateral surface of the coelom. The thoracic 

 nerves give off sympathetic branches at the dorsal margin of the ccelom. 

 The first four lumbar nerves give off spreading branches towards the limb- 

 bud. The 5th lumbar and 1st sacral nerves are but slightly developed. The 

 umbilical artery curves about the posterior tip of the coelom and sends 

 an arterial branch into the leg-bud. Below this the cardinal vein sends a 

 branch into the limb. 



Figure B. Magnification about 20 d. 



Drawing from a reconstruction of the region of the posterior extremity 

 in Embryo CLXIII; length, 9 mm.; age, about 4% weeks. The xvi (8th 

 thoracic) to the xxx (fifth sacral) spinal segments are represented. The 

 right leg and a portion of the right body-wall are shown. From the region 

 ventral to the spinal-cord unformed mesenchyme, the blood-vessels, the 

 lining membrane of the coelom, the intestine, and the urino-genital organs 

 have been removed. The 11th and 12th thoracic myotomes only are repre- 

 sented on the left side. The 9th to the 12th myotomes, costal processes and 

 spinal nerves extend ventrally in the right body-wall. Lateral and dorsal 

 branches have arisen from the spinal nerves. The sympathetic cord receives 

 branches from the thoracic and the first two lumbar nerves. The five lumbar 

 and the first two sacral nerves combine to form a plexus. From this the 

 four main nerve trunks of the posterior limb are beginning to spring. The 

 sciatic artery and vein are represented entering the limb. At the centre 

 of the base of the limb-bud the femur and hip-bone are beginning to be 

 differentiated by the formation of a dense mass in the mesenchyme. The 

 mesenchyme lying median to the limb-bud has been removed so as to expose 

 this skeletal mass and the lumbo-sacral plexus. 



