I'LATIO 1 



KXri.ANATION OF FIGURES 



1 to 9 rc'picsciit pliolographs from models of the extreme posterior end of 

 the spinal cord tioiii a 20 cm. Polistotrcma (Bdellostoma), illustrating a series 

 of three extensions of the central canal into roof plate expansions. These models 

 were prepared at a magnification of 100 diameters, and were reduced one-half 

 in i)li()t()graphing. In a few cases where certain outlines were somewhat indistinct 

 in the photographs they were strengthened with pen and ink. The roof plate 

 expansions in those models were painted lighter and appear the same in the 

 photograi)hs. Kxccpt at the posterior end of the last model, all of the models 

 of the spinal cord exhibit a marked depression and in ventral views they show 

 a pronounced indentation at the center immediately above the notochord. As 

 indicated by an arrow, the caudal direction in figures 1 to 3 is toward the left; 

 while in figures 4 to 9 it is toward the right. 



1 The most anterior of the four models seen from the right side. It includes 

 a distance of about two segments. Observe that the roof plate expansion, which 

 covers a large portion of the dorsal surface of the spinal cord, is rather low. It 

 is not known where it begins, or whether there are other outcroppings in front of 

 it as there are behind. It extends caudad some little distance on this model. 

 On the dorsal surface of the cord one dorsal or sensory root is shown, and on the 

 ventral side one ventral root is seen in entirety, being composed of several root- 

 lets. X 50. 



2 Dorsal vievvof the same modelshown in figure 1, seen from above. Note that 

 the roof expansion covers a large portion of the spinal cord. At the anterior end 

 it occupies a large central portion, then becomes gradually smaller, and at the 

 same time is confined largely to the left side, after which it gradually increases 

 in size until the posterior end of the model is reached, where it is decidedly wider 

 than at the anterior end, and extends farther over to the right side than to the 

 left. X 50. 



3 Ventral view of the cast of the central canal and extension of the same up and 

 out into the roof expansion of the same region of the cord as is shown in figures 

 1 and 2. It will be seen that the central canal and cavity of the roof plate expan- 

 sion are connected throughout. In certain places on the right side, shown in 

 white, the roof plate is solid, consisting of ependymal and connective tissue in 

 place of a cavity. For some distance posteriorly on the right side, where the roof 

 expansion is widest, there is no cavity in the roof e.xpansion. The knob-like 

 projections from the right side of the cast of the central canal represent diver- 

 ticula, and in sections through such a region there would appear to be two central 

 canals. X 50. 



ABBREVIATIONS 



C.C, central canal or cast of the same M.R., motor or ventral spinal nerve 



C.C.Ex., central canal extension into root 



roof plate expansion or cast of the R.Ex., roof plate expansion 

 same S.R., sensory or dorsal spinal nerve 



root 



