AND OF THE CAUDAL END OF THE SPINAL COlRD. l77 



and thirty-first vertebrae, tapering gradually towards its caudal end. The ventriculus 

 terminalis, which is included in the conus medullaris, expands in the medial part dorso- 

 ventrally and transversely. The upper part of this cavity, which marks the entrance of 

 the central canal, narrows slightly; the caudal end narrows sharply and forms a canal 

 which terminates blindly at the end of the conus medullaris. The wall of the ventriculus 

 terminalis consists of gray and white substance and the cavity is lined with a layer of 

 ependymal cells. The dorsal wall is thicker than the ventral wall. The filum terminale 

 extends from the caudal end of the conus medullaris, without definite boundaries, to a 

 level between the thirty-second and thirty-third vertebra?. Its caudal end is represented 

 by a slender bundle of nerve-fibers, and in its cranial portion there is a strand of ependymal 

 cells. The large coccygeal medullary vestige is situated dorsal to the thirty-third and 

 thirty-fourth vertebrae and its wall is thrown into a number of folds. At the caudal end 

 it has two processes, one extending ventrally, the other dorsally. The latter enters into 

 a rounded eminence at the caudal end of the embryo which represents a tail-bud, termed 

 by Unger and Brugsch caudal tubercle. The post-anal swelling is well developed, while 

 the coccygeal tubercle is scarcely to be made out. 



Embryo No. 199, 35 mm. Crown-Rump Length. 

 In number and development of its vertebrae embryo No. 199 is about the same as No. 

 972, description of which follows. The coccygeal vestige, however, shows greater expansion. 



Embryo No. 449, 36 mm. Crown-Rump Length. 

 Embryo No. 449 contains only 32 vertebrae, the last one being the smallest, as indi- 

 cated in figure 24. The vertebral column shows a slight ventral curve at the point between 

 the thirtieth and thirty-first vertebrae. The chorda dorsalis exhibits no convolutions at 

 its caudal end. The spinal cord narrows at a level between the twenty-ninth and thirtieth 

 vertebrae and the ventral wall of the atrophic portion presents a few folds. The remnant 

 of the medullary tube was not found in the caudal region of this specimen. There are 31 

 spinal ganglia with nerves. 



Embryo No. 972, 37 mm. Crown-Rump Length. 



Embryo No. 972 has 34 vertebrae, the last two having fused together at the center. 

 The vertebral column curves ventrally at the thirtieth and thirty-first vertebrae, the curve 

 being so sharp that the thirty-second, thirty-third, and thirty-fourth vertebrae are situated 

 in a row nearly horizontal to the trunk, as can be seen in figure 44. Cranial to thirty-first 

 the chorda dorsalis expands between the vertebrae. The caudal end is winding and broken. 



At the caudal end of the spinal cord one can recognize the primordia of the conus 

 medullaris, filum terminale, and coccygeal medullary vestige, as shown in figure 44. The 

 primordium of the ventriculus terminalis, which is included in the conus medullaris, appears 

 as a continuation of the central canal of the spinal cord without any line of demarcation, 

 and is situated at a level with the twenty-ninth vertebra. Its ventral wall is thinner 

 than the dorsal wall and shows a few small folds (fig. 44, x). The primordium of the filum 

 terminale extends from the caudal end of the conus medullaris, viz, at the level of the 

 under part of the thirtieth vertebra, to the middle of the thirty-second vertebra. It con- 

 tains an incomplete canal which is lined by a remnant strand of ependymal cells which 

 are directly continuous with the ependyma of the ventriculus terminalis. At its caudal 

 end is an ependymal strand which is directly continuous with the coccygeal medullary 

 vestige. In addition to this ependymal substance, there is a small bundle of nerve-fibers 

 along the ventral border of the filum terminale which extends into the white substance of 

 the cord above. The primordium of the coccygeal medullary vestige is situated dorsal 

 to the last two vertebra and contains a slender cavity. There are 30 spinal ganglia sup- 

 plied with complete nerves. The thirty-first ganglion has almost completely disappeared 

 on each side, leaving the nerves exposed. 



