ANATOMY OF VERTEBRATES, 



31 



inner layer - 



outer layer — 

 of fibrous capsule 



neural canal 



■ fibrous band, 

 or basis of 

 gelatinous cliorda 



cylindrical fibrous sheath, filled with simple cells containing jelly. 

 The centrums, or ' bodies of the vertebras,' are developed in and 

 from the notochord. The bases of the other elements are laid 

 down in fibrous bands, diverging from the notochord, and giving 

 the first indication of the segmental character of the skeleton. 



In Dermopteri the neu- adipose substance ' 



ral and hfemal canals are 

 formed by a separation of 

 the layers of the outer 

 division of the sheath of 



,, i -i n c\n A Transverse vertical section of vertebral column of Myxiiie. xxi. 



the notochord, ng. 22. A. 



transverse partition divides the larger portion of the neural canal, 

 lodging the myelon, from a smaller portion above containing 

 adipose tissue. In the Lancelet the substance of the noto- 

 chord, fig. 23, ch, consists of a number of circular discoid 

 or flattened vesicles, pressed one upon another within the 

 sheath, like a pile of coins in a purse ; the sheath is strength- 

 ened by a longitudinal filamentary ligament above and below. 

 Aponeurotic septa pass off, with each pair of nerves, to the 

 interspaces of the muscular segments, giving attachments to 

 the fibres. A median vertical membrane rises from the neural 



23 



cl »■* 



Li^'i' li^ & 



Diagram of anatomy of the Lancelet, Branchiostoma 



sheath, and beyond the abdominal cavity descends from the hremal 

 sheath, passing between the right and left series of myocommata. 

 The dermo-neural and dermo-hasmal spines are indicated by short 

 linear series of firmly adhering flattened cylindrical cells. The 

 next step in the skeletal tissues is shown in a pair of jointed 

 cartilaginous filaments, fig. 23, h, which bound or strengthen the 

 borders of the longitudinal oral slit, each cartilage supporting on 

 conical prominences the oral cirri (ib. f, f) : numerous carti- 

 laginous filaments strengthen the sides of the branchial cavity, ib. 

 a, with intervening fissures, not opening upon the skin. In the 

 Lamprey cartilaginous neurapophyses, fig. 24, n, n, strengthen the 

 sides of the neural canal. In the Sturgeon, fig. 25, the inner 



