36 COMPARATIVE ANATOMY 



are successively developed from its cells, and these differ both 

 chemically and physically from one another. The primary sheath 

 (so-called elastica) is first secreted by the peripheral notochordal 

 cells : the secondary sheath, which has a similar origin from 

 the so-called " notochordal epithelium," appears later, and occurs 

 in all the Craniata ; it is said not to be present in Ainphioxus, 

 the notochord of which, like that of the Tunicata, apparently 

 represents the oldest and most primitive form of this struc- 

 ture, such as is still repeated ontogenetically in Elasnaob ranch s. 

 The thick secondary sheath, which like the primary, is at first 

 homogeneous, gradually becomes fibrillar and replaces the primary 

 sheath functionally. 



From the surrounding mesoblast a skeletogenous layer is de- 

 veloped : this not only surrounds the notochord, but extends 

 dorsally to it as well as ventrally (Fig. 22). Thus a continuous 

 tube of embryonic connective-tissue is formed enclosing the spinal 

 cord and only broken through at the points of exit of the spinal 

 nerves. This stage is known as the membranous stage, and in 

 it no indication is seen of the segmentation which occurs later in 

 the vertebral axis. The cause of this segmentation is to be traced 

 primarily to the muscular-system ; and it is evident, for mechanical 

 reasons, that the segmentation of the vertebral column must 

 alternate with that of the muscular segments or myotomes. Small 

 masses of cartilage arranged metamerically later appear in the 

 skeletogenous tissue close to the notochord, and these represent the 

 rudiments of the dorsal and ventral arches and bodies or centra of 

 the vertebra: (Fig. 22, B, D, E). This is the beginning of the 

 second or cartilaginous stage of the vertebral column ; the various 

 processes (spinous, transverse, articular, &c., Fig. 22, E) are then 

 formed, and now ossification may occur (bony stage). Those 

 parts of the fibrous tissue which do not become consolidated in this 

 manner give rise to the ligaments of the vertebral column. 



During these differentiations of the skeletogenous tissue, the 

 notochord suffers a very different fate in the various Vertebrate 

 groups ; it may increase in size and persist as a regular cylindrical 

 rod, or it may become constricted at definite intervals by the forma- 

 tion of vertebral bodies, or even entirely disappear. 



All these ontogenetic stages find their exact parallel in the 

 phylogenetic development of Vertebrates, as the following pages 

 will show. 



Amphioxus, as already mentioned, apparently possesses the 

 most embryonic type of notochord. It is surrounded by a connec- 

 tive-tissue layer and is entirely unsegmented. 



In Cyclostomes a very similar primitive condition is retained ; 

 but a secondary sheath becomes developed, and cartilaginous ele- 

 ments appear in the caudal region : in the adult Petromyzon 

 these are present all along the notochord in the form of rudi- 



