in.] DEVELOPMENT. 21 



processes ; but in the more precise system of nomenclature 

 introduced by Sir Richard Owen, the one which is situated 

 highest on the arch (see Fig. 4, m), projects more or less 

 forwards as well as outwards, is usually thick and rounded, 

 and is nearly always in relation with the anterior zygapo- 

 physis, is termed metapophysis ; l the one placed rather lower 

 (Fig. 4, a), and which projects more or less backwards, and 

 is generally rather slender or styliform, is called anapophysis. 

 These, with the zygapophyses before mentioned, sometimes 

 called oblique processes, but which are rather articular surfaces 

 than true processes, are all the processes commonly met with 

 on any Mammalian vertebra. 



Development of the Vertebra. The first indication of the 

 formation of a vertebral column in the embryo is the 

 appearance of a longitudinal primitive dorsal groove in the 

 germinal membrane, the edges of which (lamina dorsales) 

 rise up and meet above, so as to convert the groove into a 

 canal. From the tissue lining this canal (uppermost layer 

 of the germinal membrane) the brain and spinal cord are 

 developed, and in its walls are formed anteriorly the cranium, 

 and posteriorly the vertebral column ; the canal itself 

 becoming the cerebral cavity and the neural canal of the 

 spine. 



In the floor of this canal, formed by a horizontal lamina 

 which separates it from another and larger, ventral or haemal 

 canal (formed by the approximation in the middle line below 

 of the lamina rentrales), a slender rod of peculiar structure 

 is developed. This is the notodwrd or chorda dorsalis, 

 around which the bodies of the future vertebrae are deve- 

 loped. In the Mammalia it almost completely disappears at 



1 It is also called " mammillary process " in some works on Human 

 Anatomy. 



