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



spinal-cord lies the notochord; on each side of this a dorsal aorta. 

 Between notochord, aortse, spinal-cord and myotomes lies a certain 

 amount of mesenchyme, which arises apparently from the ventro-median 

 surface of the myotomes. 



In the region between the dorsal margin of the myotomes, the spinal- 

 cord and the ectoderm lie spinal ganglia cells. 



Ventral to the aorta on each side the splanchnopleure is attached to 

 the axis of the embryo; ventral to the myotomes on each side, the 

 somatopleure. The latter is continuous with the amnion. The coelom 

 is surrounded by epithelium, and this is surmounted by several layers of 

 mesenchyme cells which apparently have arisen from the epithelium 

 lining the ccelom. 



Third WeeTc. 



During the third week of embryonic development marked changes 

 take place in the spinal axis of the embryo and in the adjoining somato- 

 pleure. A section throvigh the fifth thoracic segment of an embryo of 

 the third week. No. LXXVI, is shown in Fig. 17. The spinal ganglia 

 are definite, well-developed groups of cells on each side of the spinal-cord. 

 In the spinal-cord the ventral-root zone is well marked, and the first 

 ventral-root fibres have appeared. The myotomes have become flattened 

 and elongated. The median surface of each of these has become con- 

 verted into muscle cells, shown in cross-section in the figure. There is 

 a single dorsal aorta, on each side of which a Wolfl&an body has developed. 

 Dorsal to the Wolffian body lies the cardinal vein. The axial mes- 

 enchyme, which arises, at first at least, apparently from the myotomes, 

 and the mesenchyme which springs from the coelomic wall have increased 

 very greatly in amount, and have fused so as to form a common mass of 

 tissue which surrounds- the other structures. None, however, intervenes 

 between the dorso-lateral surface of the myotomes and the ectoderm. 



A vascular plexus is developing in the mesenchyme. The axial 

 mesenchyme of the caudal third of each segment has become dense. 

 This is shown at the left in Fig. 17. It represents the anlage of the 

 intervertebral disc and of the vertebral arch and costal processes. The 

 somatopleure is considerably thicker than at the stage shown in Fig. 16. 

 This increased thickness is due to an increase in the amount of the 

 mesenchyme. This mesenchyme extends for a short distance dorsally 

 between the ventral tip of the myotome and the ectoderm. The myo- 

 tomes have not yet entered the body-wall. 



In the region of the posterior limb the increase of mesenchyme be- 

 tween coelom and ectoderm precedes the formation of muscle on the 



