20 



VERTEBRATE SKELETON 



caudal halves (fig. 17, A), a matter of importance in tracing the 

 history of the parts of a vertebra. 













Ir—t. 





Fig. 17. — ^, developing sclerotomes of Sceloporus; B, of Tropidonotus (Corning). 

 a, cranial half-sclerotome; c, myocoele; cd, caudal half-sclerotome; cr, cranial half- 

 sclerotome; iv, bv, intersegmental blood vessel; m, developing blood vessel ;iw,Knotoc- 

 hord; p, caudal half-sclerotome. 



The medial portion of each sclerotome (cranial and caudal parts) 

 migrates to the side of the notochord, its cells being largely aggre- 

 gated in the V-shaped longitudinal grooves — dorsal and ventral — 



between notochord and spinal cord above 

 (fig. 18), and, in the tail between chorda and 

 blood vessels. The dorsal of these cell 

 masses on either side — two to a myotomic 

 somite, corresponding to the half sclerotomes 

 — extend upwards, tending to surround the 

 spinal cord, while below, in the tail, the 

 ventral cells enclose the caudal artery and 

 vein in the same way. Later, chondrification 

 of the parts thus formed results in the form- 

 ation of neur- and haemapophyses ; not all of 

 the mesenchyme cells are utilized in this, 

 some form ordinary connective tissue. 



Different names have been given to these 

 eight vertebral elements, based either on their 

 position or on a misconception of their fates. 

 Although adding to an overburdened nom- 

 enclature, non-commital terms are suggested 

 here. The two elements from the anterior half of each sclerotome 

 are cranineurals and cranihaemals in the following account, 



Diagram 



vertebrate embryo at begin- 

 ning of mesenchyme forma- 

 tion showing grooves (g) 

 between spinal cord (s), 

 notochord (m), and artery 

 and vein (a and v) where 

 most scleroblasts accumul- 

 ate, m , myotome; ms, 

 mesenchyme budding from 

 myotome. 



