AND OF THE CAUDAL END OF THE SPINAL CORD. 191 



figure 44 (fil. t.), and represents a prinutive filum terminale. Tliis structure extends 

 caudalward from the apex of the primitive conus medullaris at a level between the 

 twenty-ninth and tWrtieth vertebrae. In the 33 mm. embryo the cranial portion of 

 the atrophic canal and its ependymal lining disappear and the bundle of nerve-fibers 

 remains in the sheath of the dura mater. The dilated portion of the atrophic canal 

 remains as the coccygeal vestige. The process of dedifferentiation of the caudal end 

 of the spinal cord, viz, the reduction of the cranial end of the atrophic canal, advances 

 step by step, so that what appeared as a long, narrow tube in the 37 mm. embryo 

 is divided into several parts in the 50 imn. specimen, each part containing a cavity, 

 the more cranially situated part being joined to the ventriculus terminahs by a cell- 

 strand which is destined also to disappear in the course of development (figs. 44, 45, 

 and 46). The ventriculus terminalis, which had only begun in the 37 mm. speci- 

 men, has developed completely in one of 50 nmi. Its formation is the result of 

 the gradual constricting of the expanded area of the central canal which marks the 

 division between its upper wider and lower atrophic portions, and a separate cavity 

 is thus formed. Complete fusion of the margins, however, does not occur, a nar- 

 row channel being left which connects the two portions of the central canal. In 

 their thesis Brugsch and Unger have described in detail this process of reduction 

 of the central canal. In my specimens the phenomenon is first noted in the 39 

 mm embryo. In the 39, 46, 50, and 52 mm. specimens the conus medullaris, 

 ventriculus terminahs, and filum terminale are quite distinct, and the remnant 

 of the neural tube caudal to the filum terminale persists as the coccygeal medullary 

 vestige. The form of the ventriculus terminalis varies in the different specimens, 

 while that of the conus medullaris is much the same in all. In the specimens 

 above enumerated the ventriculus terminalis is situated at about the level of the 

 twenty-ninth or thirtieth vertebra, the position of both it and the conus medullaris 

 gradually becoming more cranial as the result of the fact that the growth of the 

 vertebral column becomes progressively more rapid than that of the spinal cord. In 

 these embryos, especially the 46 mm. specimen, the membranes of the spinal cord — 

 the dura mater and pia mater — are visible. At a level with the upper border of 

 the thirty-first vertebra, in the 46 mm. embryo, the dura mater may be seen to leave 

 the wall of the vertebral canal for the caudal end of the conus medullaris, which 

 marks the beginning of the filum terminale, thus forming a sheath for the latter. 

 As a rule, the coccygeal medullary vestige is on the dorsal side of the last two 

 vertebrae. It is situated in the connective-tissue surrounding the vertebrae and 

 does not adhere to the epidermis. Tourneux and Hermann discovered the caudal 

 remnant of the spinal cord in a 37 mm. embryo and termed it the vestiges medul- 

 laires coccygietis. Tourneux advanced the theory that the slightly enlarged caudal 

 tip of the neural tube is closely united in the deep layers of the skin. Toward the 

 end of the third month the spinal colunrn, developing more rapidly than the soft 

 parts, draws along the part of the neural tube adherent to it, the extreme tip of 

 which remains attached to the skin. As a result of this the terminal or coccygeal 

 portion of the neural tube becomes bent in the form of a loop, the more deeply 

 situated Umb being termed segment coccygien direct, and the more superficial one 



