18 Development of the I-iimbs, Body-wall and Back in Man 



Fourth Weelc. 



During the fourth week certain structures extend from the spinal axis 

 into the Wolffian ridge and the limb-buds. Let us consider first the 

 relation of these axial structures to the body-wall and then their relation 

 to the limb-buds. 



Fig. 20. Cross section through the fifth tho- 

 racic segment of Embryo II. x .55 d. The right 

 half of the section passes through the middle, 

 the left half through the posterior third of the 

 segment. 



coelom 

 sympathetic branch spinal i 



Fig. 21. Diagrammatic cross 

 section through the mid-thoracic 

 region of Embryo II. About 2.5 d. 

 In looking at the section the 

 si)ectator is supposed to be fac- 

 ing towards the head of the em- 

 bryo. In the background one 

 sees the fourth thoracic sclero- 

 mere with arch and costal pro- 

 cess, and at the right the inter- 

 segmental artery and the distal 

 edge of the fourth myotome. In 

 the foreground the spinal cord 

 and spinal ganglia in section, and 

 the spinal nerves of the fifth tho- 

 racic segment are shown. At the 

 left the fifth thoracic myotome 

 is shown in section. 



Fig. 20 shows the general appearance of a typical thoracic segment 

 (the fifth) in an embryo at the end of the fourth week. In Fig. 21 

 similar structures are shown somewhat more diagrammatically. Fig. A, 

 Plate II, shows a reconstruction of the axis of the body and a part of 

 the right lateral wall and the leg in the same embryo. 



The skeletal portion of each axial segment consists of a condensation 

 of the mesenchyme at the caudal third of the segment as shown at the 

 left in Fig. 20. To this skeletal tissue the term scleromere may be 

 applied. It represents the intervertebral disc, the arch or neural process 

 and the costal process of a segment of the future spinal column. The 

 general form of the scleromeres at this stage may be readily seen in 

 Fig. A. The scleromeres do not as yet extend into the body-wall. 



The general form of the myotomes may be seen in Fig. A. At the 



