. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. SKELETON. 135 The vertebrae proper arise from mesenchymatous cells, which bud off as sclerotomes (p. 102 and Fig. 111) from the developing mesothelial tissues. Some of these cells arrange themselves as a continuous envelope around the notochord (the notochordal sheath or elastica externa), while others wander inwards, be- tween the spinal cord, notochord, and muscle plates. It is to be noted that this skeletogenous tissue loses all segmental char- acter, and that the segmentation later to be seen in the vertebrae is secondar
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. SKELETON. 135 The vertebrae proper arise from mesenchymatous cells, which bud off as sclerotomes (p. 102 and Fig. 111) from the developing mesothelial tissues. Some of these cells arrange themselves as a continuous envelope around the notochord (the notochordal sheath or elastica externa), while others wander inwards, be- tween the spinal cord, notochord, and muscle plates. It is to be noted that this skeletogenous tissue loses all segmental char- acter, and that the segmentation later to be seen in the vertebrae is secondary, and is the result of the relations of myotomes and nerves. In the cyclostomes the notochordal sheath in- creases in thickness with age, and in these forms reaches its highest development. The earliest appear- ance of the segmental skeletal structures is seen as an increasing density of the mesen- chyme between the in- ner surface of each /f '4°- S«'i°" through ^ developing ver- tebral centre or the pig, showing the multiphcation myotome and the spmai of the mesenchyme cells where cartilages are to cord. These more dense arise. C, vertebral centrum ; /?, dorsal; T, ven- portions are soon con- '"â ^' '"''^^ °* ^ 'P'"^'' ""^^' ^' g^"giion of dorsal . root; iV, notochord ; R, rib; RD, R V, rami dor- verted mtO cartilage, the j^^^ ^^^ ventralis of nerve. result being a series of pairs of backwardly directed rods (the neural processes or neu- rapophyses), which tend to arch in the spinal cord. A little later similar condensations of mesenchyme take place around the notochord, a ring of this tissue occurring opposite to each pair of myotomes. This forms the rudiment of the body or centrum of the vertebra. Its subsequent history varies greatly in different groups ; and the final account cannot be written until we know more of the development, especially in the ichthyopsida. As usually described these membranous rings. Please note that
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