THE SKELETON AND MUSCLE PLATES. 353 



indicates an internal separation of the primitive vertebra 

 into two entirely distinct parts : an inner part, which forms 

 the skeleton the skeleton-plate (Fig. 95, uw, Fig. 112, wb\ 

 and an outer part, which forms the muscle the muscle^ 

 plate (Figs. 95, 96, m, Fig. 112, mp). 



The skeleton-plate is formed of the entire inner half of 

 each primitive vertebra, immediately adjoining the medul- 

 lary tube (Fig. 112, wh, wb). Its lower part, the inner 

 lower corner of the cube-shaped primitive vertebra, splits 

 up into two lamellae, which grow round the chord, thus 

 forming the basis of the vertebral bodies (wli). The 

 upper lamella forces its way between the chorda and the 

 medullary tube, the lower lamella between the chorda and 

 the intestinal tube (Figs. 68, 69, p. 276 ; Fig. 93). As the 

 lamellae of two opposite primitive vertebral pieces come 

 together from right and left and unite, a ring-like sheath 

 is formed round that particular part of the notochord. 

 From this afterwards arises a vertebral body, i.e., the 

 massive, lower, or ventral portion of the bony ring, which, 

 as a vertebra in the strict sense, surrounds the medullary 

 tube (Figs. 113-115). The upper or dorsal half of this 

 bony ring, the vertebral arch (Fig. 112, wb) arises in just 

 the same way from the upper portion of the skeleton-plate ; 

 i.e., from the inner, upper corner of the cube-shaped primi- 

 tive vertebra. The two inner, upper corners of two oppo- 

 site primitive vertebras coalesce, from right to left, over the 

 medullary tube, resulting in the closing of the vertebral 

 arch. Between each pair of vertebral arches appear, at a 

 later period, the roots of the spinal nerves, which arise from 

 the same portion of the skeleton-plate (Fig. 98, g, -y, p. 318). 



The whole secondary vertebra, which thus results from 



