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



Soon after the two dorsal aortas fnse into a single dorsal aorta inter- 

 segmental arteries are given off. From these, main branches pass be- 

 tween the myotomes, between the spinal ganglia and to the ventral 

 surface of the spinal-cord. Longitudinal anastomosing branches are 

 formed between these vessels and an extensive vascular plexus arises. 

 The blood is collected again in the cardinal veins and into the abdomi- 

 nal branches of the umbilical vein. 



The limbs and body-wall are developed in the Wolffian ridge. This 

 first appears as a thickening of the membrana reuniens along its attach- 

 ment to the axis of the body between the 4th and 26th spinal segments. 

 The limbs are developed from special bud-like projections from the 

 Wolffian ridge, the anterior extremity appearing opposite the 5th to 9th 

 spinal segments, the posterior extremity opposite the 21st to 26th. At 

 the end of the third week the Wolffian ridge and limb-buds are well 

 marked, but are without special internal differentiation. 



The body-wall is developed by an ingrowth into the Wolffian ridge of 

 processes from the myotomes, scleromeres, nerves and blood-vessels be- 

 longing to the twelve thoracic spinal segments, and a gradual differentia- 

 tion of adult structures from embryonic. Ingrowth begins during the 

 fourth week, structures essentially similar to those characteristic of the 

 adult are differentiated by the end of the sixth week. The body-wall 

 does not complete its growth to the midline, however, until toward the 

 end of the third month. 



Into the limb-buds blood-vessels and nerves extend from the axis of 

 the embryo, but neither myotomes nor scleromeres send processes into 

 the limbs. The skeletal and muscular structures of the limb are dif- 

 ferentiated from the mesenchyme of the limb-bud. Ingrowth of blood- 

 vessels precedes ingrowth of nerves. The brachial plexus is formed and 

 nerves grow into the anterior limb during the latter half of the fourth 

 week. The lumbo-sacral plexus is formed, and nerves grow into the 

 posterior limb during the first half of the fifth week. Skeletal differen- 

 tiation begins in the region of the shoulder or hip, and extends distally 

 and proximally. This differentiation immediately precedes ingrowth of 

 the nerves of the limbs. The skeletal structures serve in part to guide 

 the nerves in their distribution. Muscle differentiation immediately 

 follows the entrance of a motor nerve into a given region. From this, 

 however, it must not be concluded that a causal connection exists be- 

 tween the nerves and differentiation of muscles. " 



"See: Leonowa. Ein Fall von Anencephalle combinirt mit totaler Amyelie 

 Neurol. Centralbl. Leipsic. Bd. XII (1893), s. 218, 363. 



