450 



in the formation of the articulations between the vertebrae, but runs like a 

 thread through them. 



Development of the Spinal Column of Birds. 



In the Chick (Remak, 'Untersuchungeniiber dieEntw. 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 ; im- 

 mediately 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 vertebras (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 vertebra 

 are the result of a sort of segmentation of the motor layer ; they acquire a 

 cubical form, and are separated by clear, narrow, interspaces (metasoma- 



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 processes, which, coalescing with those of the other side, sur- 

 round the notochord and constitute the blastema of the vertebral column. 



The dorsal layer of the primitive vertebra 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. 



Rathke, ' Schildkroten,' p. 66. In birds, the centra commence as bony 

 rings which closely encircle the chorda, and lie internal to the general 

 cartilaginous mass of the vertebra. The bony substance extends inwards 

 and constricts the notochord, outwards it permeates the vertebrae. 



