liaiiiclcr Im'Iiio .loisu-vrni ral. Il sli..nl<l l.c .idled llial I (ic spin.-.l c.r.l is cri- 

 v.'l..|).'<l lin;|,tly hy a iiiciii nsica I iiiciiil.iaiic, iim.ic or less fiiscil with comicctivc 

 lissur nutsi.l.'that will Iniiii I ii.r n.'ural arcli. \\lii<-li is iiir.ily alla.'licd to the 

 iiolochonl licli.w. Iiniiicilialcly al.ovc, I lie iiicsciicliy iiic is prolirci aliiifi; rapidly 



and mij>i-al iiii; In I lie ccnlcr wlicic il will luiin I he i iiati doisal cartilaninous 



i)ar. Lilllc pro-rcss has Mcciincd in I lie loriiial ion of I he .nyoloincs al liic 

 side, and clsrw Ihmv llinv is ,,iily loos.' incscn.diynic. ■- 70. 



.')('. Similar Iransx'crsc section to li>iiiic :)."). I)iil IVoin a '-'7 iiiiii. I'oiislol rciiia 

 rnil.ivo. -nils sliiilitly lat.T sta-c shows coiisideralilc -rowt h of the tiofochord 

 and a niedian indentation on I he ventral surface of the spina! cord as tlu; result. 

 Xolc ( liat lli(>coii(litions siinoiindin^ I he dex'elopinent of t ho notocliord previous- 

 ly enmneraled undei' I he descii|)l ion of lijiiire .").') are insi ruriieiiial in assisting the 

 notochord in |.ro(lucinji the Ki'ulii.il ilatteninu' (depression ) of tlie spinal cord 

 seen in the nexl liiinre. X 70. 



.")7 'i^-ansveise section of the spinal cord of a 'iO inin Tolist ot rema embryo 

 from the same re-iion as ii.iiure .")(). It will be seen tliat the spinal cord is en- 

 closed in a membranous canal of dense connective tissue, attached below to the 

 notochord and above to the median dorsal cartilaginous bar. .\bove this there 

 ai'e developing cartilaginous rays surrounded l)y (l(>nse connect i\c tissue. The 

 d(>veloping myotonies rest against the neural arches l)oth laterall\ and dorsally. 

 'rii(> notochord has increased greatly in size and, pushing up against the soft 

 spinal ciu'd. produces the depression and ventral indent.alion of the spinal cord 

 exhibited in this figure. It should be noted that the roof plate is still ependynaa 

 and an e\|);iiision of the roof jihate could take iilaci' even in this late stage if the 



mechanical factors enumerated for the i liilla of Pet romyzon u('r(> operative 



here, 'bhe thickening of the lateral plates has about obliterated t h(> central 

 portion of the embryonic central canal, leaving only the dorsal and ventral 

 |)ortioiis. in which I luu-e is a fibrillar feltv.ork, jjiobably representing both cere- 

 bid-spinal lliiid and ependyma cilia. Reissner's fiber is visible in the ventral 

 or |iermanent central canal. X 70. 



.")S Transverse" section through the tail region of a '_'() mm. I'^ntosphenus larva. 

 It will b(» observed that the si)inal cord is fuither developed than in the 27 mm. 

 l%jlistotr(>nia einlnyo (tig. .Ki). It is apparent that the same factors are involved 

 in flattening the spinal coi-d as were enumerated for Polistotrema. The noto- 

 chord has made fully as much growth and the structures surrounding the spinal 

 cord arc the same as in Polistotrema, with the except ion t hat instead of a median 

 dorsal cartilage for the attachment of the membranous neural arch there is a 

 membranous neural si)iiie. To some extent this may reduce the dorsal resistance, 

 but on the othei- hand it may be compensated for by a greater development of 

 the myotomes above the neural arch. X 12o. 



A)iMI{KVI.VTIONS 



r..l., caudal artery M.V.C, median ventral cartil.aginous 



('.('., central canal bar 



r.//., caudal heart Mjni., myotomes 



('.v., caudal vein A'..l., membianoiis neural arch 



D.R.. dorsal cartilaginous rays .Vr.. notochord 



Ep.N., layer of e{)endymal nuclei P.M.. pia mater or meningeal mem- 



L.S., lateral veno-lymphatic sinus or brane of the young(!r stages 



anlage of the same A'./'., loof ])Iale of the central nerxous 



Mar.L.. marginal layer s\stem 

 M.D.C, median dorsal carl ilaginoiis Sj).(;., si)inal ganglion 



bar r.7'., ventral veno-lymphatic tiiink 



ir..l/., white matter 



64 



