DEVELOPMENT OF THE VERTEBILE. 



1-3 



of these intervals is very trifling in the 

 neck and in the greater part of the back ; 

 it increases in the lower third of the 

 dorsal, and still more in the lumbar 

 region. The interval between the occipital 

 bone and the arch of the atlas is consider- 

 able, and so is that between the last lumbar 

 vertebra and the sacrum. 



Fig. 12. THE SAME VERTEBRAL COLUMN AS IN 



Fia. 11, VIEWED FROM THE LEFT SlDE. 



The letters and figures indicate the several 

 vertebra. The attention of the reader is called 

 to the different curvatures of the column here 

 shown, the shape and size of the bodies and 

 intervertebral spaces, the form and transitions 

 of the transverse and spinous processes, and the 

 differences in the costal articulating surfaces. 



DEVELOPMENT OF THE VERTEBRAE. 



Earliest steps of foetal development as related 

 to the first formation of the Vertebral Column 

 and Skeleton. The first step in the formative 

 process within the ova of vertebrate animals 

 is the production on the surface of the yolk of 

 a superficial layer of organised cells, termed 

 blastoderm, or germinal membrane. In the 

 progress of development, this substance very 

 soon comes to be divided into three strata or 

 layers, which may be distinguished from each 

 other by their structural differences, and have 

 been named from their position the upper, 

 middle, and lower layers of the germinal mem- 

 brane. The upper and lower layers retain 

 throughout their simple cellular structure, and 

 take no share in the formation of the skeleton. 

 From a part of the upper layer the medullary 

 rudiments of the brain and spinal marrow are 

 derived ; while the rest of its extent is the 

 source of the cuticular covering of the body. 

 The whole of the deeper layer is taken up in 

 the formation of the epithelium of the mucous 

 membranes of the alimentary canal and its 

 dependencies. But the middle layer has a for 

 more complex and important destination, as it 

 furnishes the formative material from which 

 are developed the rudiments of the osseous, 

 muscular, and vascular systems, together with 

 the peripheral nerves, the fibrous coats of the 

 alimentary canal, and many other parts of the 

 body. 



The earliest indication of embryo-form con- 

 sists in the production of the primitive trace 

 and groove, which takes place in the part of 

 the upper and middle layers to be occupied by 

 the embryo. The deepening of this groove by 

 the elevation of its sides lays the foundation 

 of the cerebro-spinal canal or cavity, and this 

 is finally closed in superiorly by the meeting 

 and union of the elevated and approximated 



Fig. 12. 



