148 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



plate or myotome), has its own nerve (spinal nerve), its own artery 

 (inter-costal), and the basis for its skeletal tissue (sclerotome). The 

 inter-segmental septum separates one proto-vertebra or segment 

 from another. Eibs, transverse and spinous processes, are formed 

 in the inter-segmental septa. Hence an intercostal space with 

 its muscles, vessels, and nerves, with the corresponding inter- 

 vertebral structures, represents a differentiated proto-vertebra. In 

 the ventral aspect of the neck and loins, the inter-segmental septa 

 disappear. In the head nine segments are recognised, but their 

 recognition rests on observations made, not on the human 

 embryo, but on the embryoes of lower vertebrates. 



Eecent work on the segmental arrangements of the nerves 

 gives a practical importance to the number and position of the 

 primitive segments and to the part of the body which each 

 forms. Although the epiblast, hypoblast, and walls of the coelpui 

 never show any trace of segmentation in the embryo, yet clinically 

 there is evidence that each part belongs to, and was formed from, 

 one definite segment. The upper extremity grows out from the 

 5th cervical to the 2nd dorsal segments (7 segments in all), and 

 the lower from the 2nd lumbar to the 3rd sacral (7 segments in 

 all). In each limb traces of these seven segments are to be found 

 in the nerve distribution. 



Development of a Typical Vertebra, — the 6th Dorsal — 

 (1) Membranous Stage (3rd and 4th weeks). The vertebra then 

 consists of 1st a body surrounding the notochord, formed from 

 the sheath (Fig. 118 A), and 2nd a horse-shoe shaped vertebral 

 bow (Fig. 1 1 8 A and B). The bow consists of a hypo-chordal part, 

 and two lateral limbs, united by the hypo-chordal part ventral to 

 the body. 



(2) Cartilaginous Stage (Fig. 119). It commences in the 4th 

 week. The fibrous basis is transformed into cartilage except the 

 hypochordal part of the bow. It should be noticed (Fig. 118 -B) 

 that the vertebral bodies are formed round the notochord, opposite 

 each inter-segmental septum. Hence each vertebra belongs to two 

 segments. The inter-vertebral disc is situated opposite the middle 

 of a segment. The hypo-chordal part of the bow lies also in 

 the segment, and becomes included in the course of development 

 in the disc in front of the vertebra to which it belongs. 

 The lateral limbs of the cartilaginous bow meet behind (dorsal 



