180 DEVELOPMENT AND REDUCTION OF THE TAIL 



column is bent ventrally between the thirtieth and thirty-first vertebrae, forming an obtuse 

 angle and creating the typical coccygeal curve. Within the bodies of the vertebrae, from 

 the first to the twenty-ninth, the chorda dorsalis is disappearing, but a remnant still 

 remains in each intervertebral space. From the thirtieth to the thirty-fourth vertebrae it 

 continues without convolutions, but the caudal end is branched and winding, partially 

 disappearing at the dorso-caudal portion of the last vertebra close to the remnant of the 

 spinal cord. The spinal cord tapers to a point as the conus medullaris and proceeds as the 

 filum terminale from a level between the thirtieth and thirty-first vertebrae. Four por- 

 tions of the neural tube can be distinguished at the caudal end of the spinal cord: (1) 

 the sacral region of the spinal cord; (2) the conus medullaris and its contained ventriculus 

 terminalis; (3) the filum terminale; (4) a remnant. The first consists of the ependymal 

 zone, the mantle zone which contains the germinating nerve-cells, and the marginal zone, 

 as is typical for the cord as a whole. The conus medullaris extends from the twenty-eighth 

 to the thirtieth vertebrae, tapering gradually. In this region there is a large cavity, which 

 in a median sagittal section shows four walls. Through the front of the upper wall the 

 ventricle joins with the central canal of the spinal cord. The lower wall is narrow and 

 from it extend two ependymal cell-strands. The longer of these goes straight downward 

 to the first cell-group of the remnant of the medullary tube, through the axis of the conus 

 medullaris. The shorter strand can be seen at the corner between the lower and ventral 

 wall in figure 45. At the ventral wall is a diverticulum, the entrance to which appears 

 as a narrow stalk consisting of a solid cord of ependymal cells and connecting with the 

 ependymal cells of the cavity. This diverticulum is divided into two parts which are united 

 by a cell-strand — a small upper sac and a larger lower sac. The ventral walls of both 

 sacs are situated close to the surface of the conus medullaris, but do not open into it. The 

 lower part of the conus medullaris consists chiefly of nerve fibers of the spinal cord, and 

 here the central canal is entirely obliterated, leaving a long strand of ependymal cells. 

 The conus medullaris extends to a point between the thirtieth and thirty-first vertebrae, 

 and from there continues as the filum terminale, which extends to the last vertebra, skirt- 

 ing close along the dorsal side of the vertebral cohunn. At the dorsal side of the filum 

 terminale there are two remnants of the primitive neural tube. One of these is situated 

 just dorsal to the apex of the conus medullaris. It contains a slender lumen, the remains 

 of the central canal of the spinal cord. The other remnant (fig. 45, ves. m. co.) is situated 

 dorso-caudal to the thirty-third and thirty-fourth vertebrae. It is oblong in shape and 

 likewise contains a cavity, somewhat larger, which represents a remnant of the central 

 canal. Its caudal end is sharp and fuses with the caudal ligament. The latter is not so 

 distinct in this specimen as in the younger ones. 



The caudal end of the sympathetic nerve-trunk lies between the middle sacral artery 

 and vein, the three passing along the ventral side of the thirty-third and thirty-fourth verte- 

 brae, where they curve around the apex of the last vertebra. The caudal ligament forms at 

 the caudal end of the thirty-fourth vertebra and extends dorso-cranial to the coccygeal 

 vestige. The caudal portions of the sympathetic trunks unite ventral to the thirtieth 

 vertebra and become as one. After the union of these cords two additional ganglia can 

 be seen— one at the thirty-first, the other at the thirty-third vertebra. From the latter 

 the sympathetic nerve-trunk follows the midline of the vertebral column and curves around 

 the last vertebra to the dorsal side, as shown in figure 45. The condition of the dorso- 

 caudal portion of the nerve-trunk can not be clearly recognized. 



The pia mater covers entirely the surface of the spinal cord and is rich in blood capil- 

 laries. It also envelops that portion of the filum terminale containing the cell-groups 

 which connect with the ependjTiial cells of the ventriculus terminalis. The dura mater 

 traverses the wall of the vertebral canal enveloping the spinal cord and its covering of pia 

 mater. In the caudal region of the spinal cord there does not appear to be a distinct space 



