I'LATK 3 



lOXri-ANATION OK FKiUKIOS 



7, 8, and 'J rc|ucsciit laterul, dorsal, and ventral photographs of the, lourMi 

 model, which includes the extreme posterior end of the spinal cord. Note (es- 

 pecially the swelling above and below (S.T.) caused probably by an abnormal 

 sinus terminalis. From this point caudad two factors occur, which may greatly 

 modify the shape of the spinal cord. First, the notochord gradually decreases 

 in caliber and ends at {Nc. 1); second, the spinal cord has not developed a nervous 

 structure, consisting solely of ependjTiia and round undifferentiated embryonic 

 cells. As a result the spinal cord will be seen to gradually become rounded, 

 and after extending past the notochord it ends in dorsal and ventral processes 

 that become lost in the surrounding connective tissue. The posterior motor 

 roots exhibit a reduction in the number of rootlets, and they approximate each 

 other more closely. The posterior sensory roots become greatly reduced in size, 

 and the last left one has no corres]:)onding roots on the opposite side. X 50. 



ABBREVIATIONS 



M.R.{1), last motor or ventral spinal S.R. (1), last sensory or dorsal spinal 



nerve root nerve root 



Nc. (1), posterior end of Polistotrema S.T.. sinus terminalis 



notochord shown in model 



platp: 4 



EXPLANATION OF FIGURES 



10 to 23, show 14 transverse sections through the posterior spinal cord region 

 of a 20 cm. Polistotrema (Bdellostoma), the same as was modelled and shown in 

 figures 1 to 9. The outlines were all drawn with the aid of a Leitz-Edinger draw- 

 ing apparatus, using a magnification of 140 diameters and were reduced one- 

 half in reproduction. They are numbered consecutively from anterior to posterior. 

 Figures 10 to 14 pass through what has been designated as the first roof plate 

 expansion; figures 15 to 17 through the second roof plate expansion; figure 20 

 through the abnormal sinus terminalis; and figures 21 to 23 at various intervals 

 through the extreme posterior end of the spinal cord, which has failed to develop 

 any nervous structures, and has not to any extent been flattened in its develop- 

 ment by the growth of the notochord. The enormous concavity seen on the ven- 

 tral surface of spinal cord in figures 11 to 14 is to a large extent an artifact due to 

 the preparation of the series. It should be noted that the cavities of the roof 

 plate expansions are full of a fibrillar feltwork, for the most part coagulated 

 cerebro-spinal fluid, that their walls are moderately expanded by it, and their 

 cells are sufficiently granular to suggest a secretary function. 



10 is from the most anterior section of this series. The entire neural arch, 

 median dorsal cartilaginous bar, and a portion of the notochord are included in 

 this figure. The so-called first roof plate expansion covers a large area of the 

 central portion of the spinal cord; its cavity is in communication with the cen 

 tral canal. The ependymal walls of both the cavity and central canal are com- 

 posed of several layers of cells. Motor roots, motor cells, substantia gelatinosa 

 cells, and blood vessels are to be seen in transverse section. X 70. 

 {Continued on pnge, 50) 

 48 



