Charles Eussell Bardeen and Warren Harmon Lewis 19 



right the 11th and 13th thoracic and the 1st and 3d lumbar myotomes 

 are viewed from the side and slightly in front. At the left a portion 

 of the median surface of several myotomes may be seen. In Fig. 20 

 two myotomes are shown in cross-section. The myocoel has disappeared. 

 The median surface of each myotome has been entirely converted into 

 musculature. The lateral surface and ventral and dorsal tips are cov- 

 ered by epithelium. A certain portion of the dorsal surface of the 

 thoracic myotomes is, however, converted into musculature. This is 

 shown in the myotomes at the right in Fig. A. The thoracic myotomes 

 extend for a short distance into the body-wall. In Fig. 20 this is shown 

 in a cross-section. In Fig. A the projecting tips of the myotomes may 

 be seen through tlie membrane lining the coelom. 



The thoracic spinal nerves project for a shorter distance into the body- 

 wall than do the myotomes. Each is divided at the dorsal margin of the 

 coelom into two portions, a median which becomes the sympathetic 

 ramus, and a lateral which extends outwards between the median surface 

 of the myotome and the lining membrane of the coelom and becomes the 

 ventral trunk of the spinal nerve (see Figs. A, 20 and 21). 



From the aorta intersegmental arteries arise and send branches dor- 

 sally toward the spinal ganglia, laterally between the myotomes and 

 ventrally into the body-wall (see Fig. 21). A considerable vascular 

 plexus is developed in the mesenchyme. The general relations of the 

 latter are shown in Fig. 20. The venous blood is collected in the cardinal 

 veins and in branches of the umbilical vein. 



The general relation of the formed structures of the axis of the body 

 to the leg-bud at the end of the fourth week are shown in Fig. A, 

 Plate II. The limb-bud lies opposite the five lumbar and the first sacral 

 segments. The coelom extends to a point opposite the first sacral seg- 

 ment, but in the region of the limb it does not extend as far dorsally as 

 in the thoracic region. From the model represented in Fig. A, several 

 of the myotomes of the left side, the axial mesenchyme, the aorta, the 

 left cardinal vein, the intestines and urino-genital organs have been 

 removed. A ^^ortion of the right cardinal vein and a portion of the 

 right umbilical artery are shown, reduced in size for the sake of clear- 

 ness. The umbilical artery curves about the distal extremity of the 

 coelom. From the umbilical artery a branch passes into the limb-bud. 

 Veins pass from the limb-bud into the cardinal vein. The blood-vessels 

 of the limb exist at this time in the form of an irregular sinusoidal 

 plexus. 



The second, third and fourth lumbar nerves may be seen sending 

 spreading bundles of nerve fibres into the dense tissue of the limb, dorsal 



