AND OF THE CAUDAL END OF THE SPINAL CORD. 195 



misnomer. Sometimes it appears as a rounded eminence and was therefore termed 

 caudal tubercle by Unger and Brugsch. 



(8) The tail-like appendage that occasionally persists in adults may possibly 

 -be explained as a persistent caudal tubercle that did not undergo the normal reduc- 

 tion. It must be granted, however, that in none of the cases reported has osseous 

 or cartilaginous tissue been found. 



(9) When the embryo reaches 30 to 35 mm. the tail has usually entirely dis- 

 appeared, although the time of disappearance is quite variable. Thus, the tail 

 was found to have disappeared in the 24 mm. embryo, while in one 39 mm. long 

 it still persisted. 



(10) In embryos above 40 mm. in length that have lost the external tail I 

 have designated as the internal tail the portion caudal to the thirtieth vertebra, 

 for three reasons: (o) the curve of the vertebral column occurs at the thirty- 

 first and thirty-second vertebrae; (h) below the twenty-ninth vertebra the spinal 

 ganglia disappear at about the same time as the external tail; (c) the sympathetic 

 ganglion strands unite between the thirtieth and thirty-first vertebrae. 



(11) The disappearance of the canal of the caudal gut had already begun in a 5.5 

 mm. embryo and in a 6.5 mm. specimen the caudal gut had become converted into a 

 long cell-strand, excej)! for a short caudal portion. In embryos 7.5, 8, and 9 mm. the 

 remnant of the caudal gut, inclosing a small cavity, was still found in the end of the 

 tail. In those 10 mm. and older the caudal gut had entirely disappeared. 



(12) In the very youngest specimens the medullary tube reaches to the extreme 

 tip of the tail. 



(13) In those 11, 12, and 15.5 mm. long the medullary tube can be divided 

 into two parts at the level of the thirty-second vertebra: a cranial part, having 

 a wide central canal, and an atrophic caudal part with a narrow canal. This 

 distinction becomes quite marked in the 15.5 mm. specimen. The canal at the 

 junction of these two parts is slightly enlarged transversely and constitutes the 

 primordium of the ventriculus terminalis. 



(14) The atrophic portion of the spinal cord gradually becomes more slender, 

 although its caudal end remains unchanged for some time or in some instances 

 shows temporary enlargement. It later subdivides, the cranial end forming the 

 cell-strand of the filum terminale and the caudal end developing uito the coccygeal 

 medullary vestige. 



(15) The caudal end of the wider part of the spinal cord develops into the 

 conus medullaris and its lumen constitutes the ventriculus terminalis. 



(16) In the 46 mm. embryo the ventriculus terminalis is perfectly developed. 



(17) The conus medullaris and the ventriculus terminalis recede cranialward 

 as the result of two processes: (a) the growth of the vertebral column, which is 

 more rapid than that of the spinal cord ; (6) the degeneration of the gray substance 

 which forms the upper wall of the ventriculus, thus causing the cavity to enlarge 

 and gradually move upward while its caudal end narrows. 



(18) The extent of the coiling of the chorda dorsaUs in its various stages 

 indicates the extent of fusion of the last primitive vertebrae. 



