PLATE 9 



EXPLANATION- OF FIGURES 



42 to 44 Three tran.sv(M-sc sections through the iiKMliiUa of an Is day Petromy- 

 zon embryo. i)a.ssinp; through the V. VIII. and X ganglia, and for the sake of com- 

 parison preserving the same order or arrangement as was used for the earlier 

 embryos. A slight increase in the white matter is to I.e noted for the lateral and 

 ventral plates over the \2 day series; but little, if any change has taken place in 

 Ihe central canal, unless possibly the central jmrtion has increa.sed slightly in 

 width. Ab.solutely no further expansion of the roof plate has occurred. Since 

 sections through the medulla of a 14 and 10 flay series jiresented about the same 

 appearance as figures 42 to 44 none were figured. X 12."). 



45 Tran.sverse section through the ce|)halic portion of the spinal coni fn.ni 

 the same series as figures 42 to 44. Dbserve especially tiie begiiming of the dorsal 

 closure of the central canal (CCC). showing the central canal to consist of dor- 

 sal and ventral cavities and a central scam, strongly resembling the stage wiieii 

 it first api)eared in the spinal cord. This <lorsal closure of the embryonic central 

 (•anal begins in the anterior portion of the spinal cord much earlier than the cor- 

 responding ventral closure of the embryonic central canal in th«- nu'dulla, occur- 

 ing in my .series of 14 and In days. It is obviously caused by the ingrowth of 

 the lateral plates due to the great increase in the ninnb(>r of nerve fibers. X 12.'). 

 4(1 to 4.S Three tran.sver.se sections through the same region of the medulla of 

 a 20 day Petromyzon as is shown above in figures 42 to 44 for the l.S day embryo. 

 It will be .seen that many noticeable changes have taken i)lace. du«' primarily 

 !.• a marked increa.se of nerve fibers in the lateral and ventral plates, and to a 

 slight increa.se in the number of cells in the lateral plates. The shape of the 

 medulla has become more compressed (flattened out in a dorso-ventral plane. 

 Unquestic.nably the increase of fibers in the lateral plates has occurred largely 

 in the median and ventral portions. Note the result on the embryonic central 

 canal, which has been completely closed, except for a small dor.sal triangular 

 cavity (C.C) the early fourth ventricle. Its roof plate, however, has expanded 

 somewhat. Ob.serve that true ependymal cells are l)eginning M take on form 

 .ibout the ventricl(>, and those in the roof j)latemay soon assume a secretory func- 

 lioii, if tlirv .nc not already active. .Vlso the blood vessels have become more 

 .ilniiidaiii outside of the medulla, especially in the region of the roof plate, which 

 would make infiltration and diffusion thniugh the mof plate into the ventricle 

 easy. X 12.".. 



lit From ;i transverse section through the anterior portion of the s|)inal cord. 

 Obs.MVe tile great increase in fibers in the lateral and ventral plates, and n<ite 

 that tile (left-like embryonic central canal has become entirely closed, but for 

 a small portion, which will remain as the adult central canal. It is obvious that 

 the two ])()rtions of the emliryonic central canal which persist in the medulla 

 and t lie spinal cord are the opposite, being the dorsal in the medulla :ind the ven- 

 tral in the cord. X 12."). 



.')0 to 53 Four transver.se sections through the medulla of a 2(j day Petromy- 

 zon embryo, taken through the V. VIII, and X ganglia, and through a region 

 liehind the fourth ventricle where the central canal is j^assing ventral to assume 

 its characteristic position in the spinal cord. A comiiari.son with the 20 day 

 series above (figs. 4(5 to 4S) will demonstrate a marked increase in the size of the 

 medulla and the fourth ventricle, and a greater expansion and convexity of the 

 (Continued on page 62) 

 61 



