252 



VERTEBRA 



discs. Bardeen did not observe this stage in the human embryo. Within the 

 discs the notochord is enlarged and afterwards converted in each along with the 

 surrounding tissue into the nucleus pulposus. Within the bodies, on the other 

 hand, the chorda becomes constricted and ultimately disappears. 



The neural arches retain their primitive position, and while the costal processes 

 are extending between the my o tomes to form the membranous ribs, a transverse 

 process appears on each side in the angle between it and the neural process, 

 opposite the attachment of the scleromere to the notochordal sheath. A 

 chondrogenetic centre appears in each side of the primitive arch, and another in 

 each costal process. The arch becomes joined to the body, and the cartilaginous 

 vertebra is completed. The arches, however, remain for a long time open 

 (figs. 306, and 234, 268, pp. 186, 214). It is not till the fourth month that they are 



rib 



spinal ganglion 



scapula 



Y scapula 



humerus 



vein thy. \ thy. vein 

 sternum 



FIG. 306. SECTION OF A HUMAN EMBRYO OF 30 MM. Photograph. (T. H. Bryce.) 

 s.p., spinal cord; n.c., cartilages of neural arch still separate; thy, thy, thymus. 



closed in over the cord to complete the spinal canal. The articular processes are 

 produced by the growth of cartilaginous rudiments, backwards and forwards, 

 from the neural processes into the intervening layer of tissue. The primitive 

 membranous plate at an early stage becomes much thickened ventrally. This 

 thickened band corresponds to the hypochord rod of lower forms. It is not 

 a separate structure at any time (rat, Weiss ; man, Bardeen), except in the 

 case of the atlas, in which, loosened from the body, it persists as the ventral bar 

 of that bone. It becomes chondrified by two lateral centres (Weiss). The body 

 of the atlas remains free from the bases of the arches, and is secondarily fused with 

 the axis as its odontoid process. 



Ribs and sternum. Each vertebra is provided with a rib-process. In the 

 blastema stage there is no separation between rib and primitive vertebra, and the 



