600 ON THE THEORY OF THE .VERTEBRATE SKULL 



and further from the axis, until at length it appears as a common trunk for both, 

 and therefore may easily deceive one into supposing, that the rib is given off from 

 the base of the crus of the arch, and is a process from it." 



2. ''' Lacertilia (Rathke, 'Ueber die Entwick. d. Schildkroten,' 1848, pp. 65-67). 

 — In the lizards (as in the snakes), the osseous centra of the vertebrae appear as 

 rings, which do not so closely embrace the notochord (as they do in fishes and 

 Batrachia), being separated from it by a layer of cartilage. 



3. Chelonia (Rathke, 'Schildkroten,' p. 65-67). — In the Cheloitia two bony rings 

 arise, the one on the outer surface of the caitilaginous basis of the centrum, the 

 other close to the notochord ; the rings thicken and eventually coalesce ; the 

 ossification of the arches takes place quite independently of that of the centrum. 

 The notochord takes no essential part in the formation of the articulations between 

 the vertebra, but runs like a thread through them. 



Development of the Spinal Column of Birds. 



In the Chick (Remak, ' Untersuchungen uber die Entw. die Wirbelthiere,' 1855). 

 — The upper and middle (sensory and motor) layers of the germ coalesce to form 

 the axial plate (primitive streak) ; the lateral halves of this plate thicken and 

 leave between them a groove, the primitive groove ; immediately below the groove, 

 and parallel with it, the notochord appears in the axis of the motor layer. The 

 sensory layer of the axial plate is now the medullary plate ; from it the nervous 

 centre is developed ; the motor layer is termed by Remak the ' Urwirbel-platte,' — 

 the primitive vertebral plate. 



The primitive vertebrje (Urwirbel, somatomes) first appear in the dorsal part 

 of the embryo, as opaque portions of the substance of the primitive vertebral 

 plates, which extend from the sides of the chorda into the lateral plate, or that 

 thickened part of the motor layer with which the primitive vertebral plates are 

 immediately continuous. These primitive vertebrae are the result of a sort of 

 segmentation of the motor layer ; they acquire a cubical form, and are separated 

 by clear, narrow interspaces (metasomatomes). 



At the beginning of the third day the ventral surface of each primitive vertebra 

 has an almost square shape with rounded angles ; the transverse section is no 

 longer square but three-sided, the upper and outer faces having merged into one 

 convex face ; the surface turned towards the medullary canal is four-sided and 

 a little concave. 



The inferior internal edge of the primitive vertebra grows out towards the 

 notochord, and having reached its outer side, it divides into two lamellar pro- 

 cesses, which, coalescing with those of the other side, surround the notochord and 

 constitute the blastema of the vertebral column. 



The dorsal layer of the primitive \ertebra becomes converted into muscle, and 

 forms a segment of the dorsal muscles ; the anterior portion of its substance, 

 beneath this, becomes the spinal ganglion ; the posterior, the rudiment of the 

 neural arch and rib. The latter extends backwards beyond the boundary of its 

 primitive vertebra into the region of the next, so that it appears to be divided by 

 the clear line of separation into a larger anterior and smaller posterior portion. 



The axial portion of the vertebral column does not become segmented in 

 correspondence with the divisions between the primitive vertebrae, but midway 

 between them, so that the lines of separation between the primitive vertebrae 

 correspond with the centres of the permanent vertebrae, each of which may thus 

 be said to be formed by the coalescence of the posterior half of the axis of one 

 primitive vertebra with the anterior half of the next following. 



