AND OF THE CAUDAL END OF THE SPINAL CORD. 181 



between the pia mater and dura mater and hence the arachnoid membrane is not visible 

 at this point. A short distance from this, however, where the membranes envelop the 

 conus medullaris, there is a marked space between the two membranes and here the arach- 

 noid can be fairly well made out, forming a fibrous network of embryonic connective- 

 tissue. At the level of the caudal third of the thirtieth vertebra where the filum terminale 

 begins, the dura mater fuses with the pia mater and the two become separated from the 

 wall of the vertebral canal and extend caudalward. The second group of cells, which 

 lies caudo-dorsal to the thirty-third vertebra, does not seem to be covered by the pia mater 

 or dura mater, these membranes having disappeared a short distance above. 



Embryo No. 1656, 67 mm. Ceown-Rump Length. 



There are 34 vertebrae in embryo No. 1656, the last being the smallest. At the thirty- 

 first and thirty-second the vertebral column shows a ventral curve, the angle being sharper 

 than in the younger specimens. The vertebrse are separated by embryonic tissue which 

 is to develop at a later stage into intervertebral fibro-cartilage. This separation becomes 

 progressively more marked above the thirtieth vertebra. Between the vertebrae which 

 still lie close together is a small sjiace where the chorda dorsalis coils as it emerges from the 

 vertebral bodies in the median line. Several of these coils can be seen in figure 46, which 

 is a profile reconstruction through the caudal end of the embryo. The blood-vessels enter 

 the vertebral bodies from the ventral and dorsal side. 



In the conus medullaris there are two medullary ventricles. The more cranially 

 situated one is somewhat smaller than the other, measuring 0.55 by 0.25 by 0.33 mm. 

 Its form, as seen in the sagittal plane, can be recognized in figure 46 (vent. t. cran.). The 

 lower cavity is oblong in shape, measures 1.1 by 0.3 by 0.36 nmi., and presents a canal- 

 like appendage 1.7 mm. in length, as seen in figure 46 {Append.). This appendage tapers 

 to a point and continues as a cell-strand. Toward the caudal end of the strand, in the 

 path of the filum terminale, are two small groups of cells which represent the remnants 

 of the ependymal cells of the medullary tube (fig. 46, Re. epend.). 



The phenomenon of dedifferentiation at the caudal end of the spinal cord is well 

 shown in this specimen. The appendage of the lower cavity was a complete ventriculus 

 terminalis at the first stage ; the main body of the cavity was a complete one at the second 

 stage, and the upper cavity is the ventriculus terminalis at the present stage, thus showing 

 a progressive upward trend. The gray substance which primarily existed around the ven- 

 triculus terminalis has now disappeared as the result of degeneration, and the caudal end 

 of the central canal has gradually enlarged. The caudal end of the lower cavity, however, 

 is becoming gradually narrow because the caudal portion of the conus medullaris, which 

 contains the ventriculus terminaUs, has also gradually become atrophied and lost its cell- 

 like substances. The septum between the two ca\aties is a remnant of the gray substance 

 of the spinal cord, in which the degeneration is not yet complete. 



The filum terminale follows a downward course from the end of the conus medullaris 

 and nerve-fibers can be recognized as far down as the caudal portion of the thirty-second 

 vertebra. In the caudal region are found two cell-groups representing remnants of the 

 neural tube ; one, which lies between the thirty-second and thirty-third vertebrae, contains 

 no lumen, and the epithelial cells are undergoing degeneration. The other is situated 

 dorsally between the thirty-third and thirty-fourth vertebrae and incloses a small lumen. 



The membranes of the spinal cord are more easily made out in this specimen than in 

 the younger ones. The dura mater is separated from the periosteum of the vertebral 

 bodies, especially at the ventral wall of the vertebral canal, by a dense plexus of blood- 

 vessels, connective-tissue, and small spaces. This separation occurs at a level between 

 the twenty-seventh and twenty-eighth vertebrae, and the dura mater becomes adherent 

 to the conus medullaris between the twenty-eighth and twenty-ninth vertebrae, following an 



