AND OF THE CAUDAL END OF THE SPINAL CORD. 179 



to the wall of the vertebral canal as far as the midlevel of the thirty-first vertebra, at which 

 jinint it leaves the wall and unites with the caudal end of the conns medullaris. This 

 portion constitutes the primordiuni of the bursa dura:- matris. After the dura mater reaches 

 the conus medullaris it envelops the pia mater quite closely, both following a caudal course 

 and forming a sheath for the filum terminale. The point at which these membranes 

 terminate can not be definitely decided. It is probable that the pia mater extends nearly 

 to the end of the filum terminale between the thirty-third and thirty-fourth vertebrae. 

 The fibers of the dura mater appear to enter into the caudal and dorsal portions of the last 

 vertebra. 



Embryo No. 184, 50 mm. Crown-Rump Length. 



Embryo No. 184 has 34 vertebrae, the last one being the smallest, as is indicated in 

 figure 28. At the thirty-first vertebra the column presents a ventral curve, bringing the 

 thirty-second, thirty-third, and thirty-fourth vertebrae in about a horizontal row and at 

 right angles with the main column. The chorda dorsalis is disappearing in the 29 upper 

 vertebral bodies, but at the thirtieth and below there is no change from the earlier stages, 

 except that the chorda is relatively more slender. Its caudal end is bent caudo-dorsally 

 before terminating; from this point the caudal ligament takes its origin. The middle sadral 

 artery at this stage is a relatively delicate vessel, running from the ventral to the dorsal 

 side of the vertebral column, and curving about the apex of the thirty-fourth vertebra. 

 Its branches are plexiform, and in their meshes are groups of cells resembling neuroblast 

 cells. The caudal end of the spinal cord contains a large cavity representing the ventric- 

 ulus terminalis at a more advanced stage of development. The upper end of this cavity 

 connects with the central canal of the spinal cord; its lower end ternxinates in two horns, 

 the dorsal one of which is a blind pouch; the ventral horn is united with the caudal rem- 

 nant of the spinal cord by a strand of ependymal cells and many transverse folds. The 

 caudal remnant of the spinal cord consists of three separated portions. The first, which 

 is attached to the caudal end of the ventriculus terminalis by an ependymal cell-strand, 

 lies between the thirtieth and thirty-first vertebrae. This portion is embedded in nerve- 

 fibers. As in younger specimens, it incloses a narrow cavity interrupted about midway. 

 The second portion of the remnant is situated between the thirty-first and thirty-second 

 vertebrae and leans to the dorsal side of the filum terminale. It also contains a small 

 lumen. The third and largest portion is situated at the level of the thirty-third vertebra ; 

 its cavity is larger than the others and its caudal end enters into the caudal ligament. 



The pia mater envelops the spinal cord and contains blood capillaries. It traverses 

 the course of the filum terminale, completely inclosing it, and appears to reach the dorsal 

 portion of the thirty-third vertebra, at which point the filum terminale ends. The dura 

 mater also covers the spinal cord over the pia mater. At the caudal end of the conus 

 medullaris, about the thirtieth vertebra, the dura mater adheres closely to the pia mater. 

 At the dorsal side of the thirty-third vertebra the fibers of the dura mater merge with the 

 fibers of the caudal ligament. 



This embryo has .31 spinal ganglia on the right side and .30 on the left. The last 

 ganglion on either side is very small, being in process of retrogression. The right thirtieth 

 and thirty-first ganglia and the left thu-tieth are not located between the vertebrae, but 

 at the dorsal side of the upper vertebral bodies. 



Embryo No. 448, 52 mm. Crown-Rump Length. 



A profile reconstruction of the caudal end of embryo No. 448 is shown in figure 4.5 

 and a more diagranmiatic sketch is shown in figure 29. The embryo has 34 vertebrae, the 

 last of which is only three-fourths the size of the thirty-third. The last three have begun 

 to fuse, so that a section cut through the axis of the vertebral column shows one large 

 vertebral body representing the three vertebrae, as shown in figure 45. The vertebral 



