SKELETON. 



135 



The vertebrae proper arise from me_senchymatous cells, which 

 bud off as sclerotomes (p. 102 and Fig. 1 1 1) 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 vertebras 

 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 J* G ' I4 ' Section through a developing ver- 



tebral centre of the pig, showing the multiplication 



myotome and the Spinal of the mesenchyme cells where cartilages are to 

 COrd. These more dense arise. C, vertebral centrum; Z>, dorsal; F, ven- 



portions are soon con- tral roots of a s P inal nerve ; ^ S an g lion of dorsal 



root; A r , notochord ; A', rib; RD,RV, rami dor- 



verted into cartilage, the salis and 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 



