166 Development of Thoracic Vertebra) in Man 



This figure represents a horizontal section passing through several 

 spinal ganglia, myotomes and neural processes. The last may be seen 

 extending gradually into the area opposite the myotomic septa, but they 

 still cover the whole posterior half of the median surface of the myotome 

 in the region of the section. The processes are connected by membranous 

 thickenings of the mesenchyme of the anterior half of each segment. These 

 membranes may be called interdorsal membranes. They correspond to 

 the interdorsalia of elasmobranchs. Jb'igs. 12, 13, 15 and 16, Idr. M., 

 represent these membranes. A line drawn from A to B in Fig. 12 would 

 pass through an area corresponding to that of the section represented 

 in Fig. 4. 



In the region where the neural and costal processes spring from the 

 primitive disks membranous septa are likewise differentiated from the 

 anterior halves of the sclerotomes. These septa serve to unite the suc- 

 cessive disks. Each is continuous posterially with a dense tissue which 

 strengthens the primitive disk and anteriorly it extends into the neural 

 and costal processes. The relations of these interdiscal membranes are 

 shown in Figs. 3, 11, 12 and 13, Ids. m. Since at this period structural 

 outlines are by no means sharp, the figures based upon wax-plate recon- 

 structions must be taken as semi-diagrammatic. A line drawn from 

 c to d^ in Fig. 12 would represent essentially the plane of the section 

 shown in Fig. 3. 



During the development of the interdiscal membranes the primitive 

 disks become hollowed out on the posterior surface. A comparison of 

 Fig. 2 with Fig. 3 demonstrates this. The perichordal sheath meanwhile 

 is developed in a ventrodorsal direction so that the area between the 

 primitive disks becomes divided into right and left halves. Figs. 11, 13 

 and 7 all show this. Each lateral area is filled with a lightly staining 

 mesenchyme which is continuous ventrally and dorsally with the tissue 

 surrounding the spinal column. 



Fig. 17, Plate V, represents a sagittal section cut slightly obliquely 

 through an embryo 12 mm. long. In the region where the chorda 

 {Ch. d.) is cut, the primitive disks may be seen united by a fairly dense 

 tissue, the perichordal septum (Sptm.). Posterior to this region the 

 section passes to one side of the chief axis of the embryo. The inter- 

 vertebral disks may here be seen separated by a lighter tissue and in the 

 more posterior portion of the section, which passes still more lateral to 

 the chordal region, the tissue between the disks is seen to be continuous 

 with that surrounding the spinal column. In this region the interdiscal 

 membrane (Ids. m.) is seen anterior, the primitive disk posterior to the 

 fissure of v. Ebner {F. v. E.). 



