2 1 8 ELEMENTAR Y ANA TO MY. [LESS. 



The fundamental facts of development (noticed in the Second 

 Lesson) must also be borne in mind : how the dorsal laminae 

 ascend and form the neural canal, and how the ventral 

 laminae descend to form the trunk. Also, how each of these 

 ventral laminae splits longitudinally on each side into an outer 

 plate, forming the body-wall (bones, muscles, nerves, &c.), 

 and an inner plate, forming the alimentary canal and its ap- 

 pendages the split laminae being united respectively at their 

 ventral ends. 



FIG. 187. Diagram of the development of the Trunk and Its Skeleton, as shown 

 in a section made at right angles to the trunk's long axis, nc, neural canal ; 

 ex, epaxial cartilages ascending to surround it ; px r paraxial cartilages de- 

 scending in the plate, or layer (Ive), external to pp t the pleuro-peritoneal cavity ; 

 Ivi, internal plate of the split ventral lamina. 



Between these outer and inner plates is a space (to be 

 treated of in the Eleventh Lesson) called the pleuro-peritoneal 

 cavity, which thus separates the external tube of the trunk 

 from the alimentary tube internal to it. 



Not only the alimentary tube, but also the heart and the 

 roots of the great vessels which proceed from it, are, as we 

 shall hereafter see, placed within the inner wall of this 

 pleuro-peritoneal cavity. 



6. The central axis of the skeleton is evidently the noto- 

 chord with the structures immediately investing it, together 

 with those which generally by ossification replace or en- 

 croach upon it later in the development of the individual. 



In one single form, the Lancelet, and in that only, the noto- 

 chord extends forwards to the front end of the body, and 

 much beyond the spinal marrow, the anterior end of which 

 is the only representative of the brain. In all other forms 

 the notochord stops short behind the pituitary fossa or the 

 part representing it. 



Nevertheless, median parts in the same axial line with the 

 notochord may be formed, as the pre-sphenoid, which is as it 

 were a supplemental piece of the axis added on later to the 

 primitive termination of the skeletal central axis. 



