324 MR. J. S. BUDGETT ON THE STllUOTUEE 



Tlie Vertehral Column. 



The notochord is at this stage of even calibre throughout the greater part of its 

 length, tapering suddenly, however, anteriorly and gradually posteriorly towards 

 its termination, where it shows a slight upward tendency (PI. XXXIII. figs. 4 

 5, 6, Ch.). 



The structure of the notochord in Polypteriis is very similar to that of Lepidosteus 

 and the Teleosteans. 'J'he meshes between the large vacuoles enclose here and there a 

 nucleus. The vacuoles towards the sheath become smaller and the nuclei more 

 numerous, and pass rather suddenly into a definite nucleated epithelium lining the 

 sheath of tlie notochord (PL XXXIV. fig. 10, Ch.ep.). The sheath itself is very 

 thick and almost structureless, being from ^jj to i\- the thickness of the diameter of 

 the vacuolated portion of notochord. A very delicate membrana elastica bounds the 

 sheath externally (PI. XXXIV. fig. 10, m.el.e.) and separates it from the connective 

 tissue surrounding the notochord with which the septa between the muscles are 

 continuous. 



At the junction of these septa with the notochordal sheath are found on either side 

 three metameric series of cartilages — a dorsal row of stout pyramidal masses forming 

 the bases of the neural arches with which they are continuous (PL XXXIII. figs. 4, 



5, 0, «& PL XXXIV. fig. 2, n.jjr.) ; a lateral row of small pyramidal masses forming 

 the foundations for the transverse processes and lateral ribs (PL XXXIII. figs. 4, 6, 

 & PL XXXIV. fig. 2, Lpr.) ; and the ventral row of still smaller nodules forming 

 the foundations of the ventral series of ribs (PL XXXIII. figs. 4, 5, 6, & PL XXXIV. 

 figs. 2 & 10, v.pr.). 



All these cartilages rest directly on the notochordal sheath, and in each metamere the 

 three masses lie in nearly the same transverse plane, so that one section often passes 

 through all three structures. 



Neither the pyramidal bases nor their neural prolongations are fused across the mid- 

 line, except in the tail-region. The bases are quite separate, while the upper ends of 

 the two halves of the neural arch curve backwards, come into close contact with one 

 another, and carry the lower end of the median dorsal spine (PL XXXIII. tigs. 4, 5, 



6, d.sjp.). 



In the anterior region of the vertebral column all three series are well-developed. 

 In this region the lateral series are continued outwards along the septa, between the 

 dorso-lateral and the ventro-lateral muscles, forming complete rib-like structures 

 (PL XXXIII. fig. 4, l.jjr.). 



The ventral series are also well-developed and pass outwards between the ventro- 

 lateral muscles and the kidneys (PL XXXIII. fig. 4, & PL XXXIV. figs. 2 & 10, v.j)/:). 

 At this stage they do not in the anterior region reach the peritoneum. 



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