192 DEVELOPMENT AND REDUCTION OF THE TAIL 



segment coccygien reflechc. In mj' specimens I did not find such to be the case, 

 nor was it noted by Unger and Brugsch. I beheve, therefore, that the condition 

 noted by Tourneux is of rare occurrence. In the 39 nun. embryo may be seen a small 

 papiUiform tail, at the root of which is a group of cells representing the remnant of 

 the spinal cord. The caudal end of the coccygeal medullary vestige appears to 

 adhere to the epidermis, but in reality does not, although Tourneux and Hermann 

 found that it did adhere in their case. 



Concerning the development of the coccygeal medullary vestige from the rem- 

 nant of the neural tube, I am led to the following conclusions: 



(1) The expanded caudal end of the neural tube in an embryo in which the 

 tail has disappeared is the primordium of the coccygeal vestige (figs. 40, 42, and 43). 



(2) In addition to the coccygeal vestige there frequently occurs a similarly 

 formed epithelial sac situated in a more cranial position. 



(3) The caudal end of the coccygeal vestige merges into the caudal hgament, 

 as believed by Brugsch. 



(4) In the younger specimens the fibers which persist as the filum terminale 

 always he ventral to the epend>Tnal cells, which become the coccygeal vestige. 



(5) The middle sacral artery and vein extend to and curve around the apex 

 of the coccygeal vestige. 



(6) Only in rare cases is the coccygeal vestige lacking. In my entire series 

 of specimens, from 4 to 125 imn., in only one did I actually fail to find it (fig. 24). 



(7) In specimens from 4 to 100 mm. the coccygeal vestige is not adherent to 

 the epidermal layer of the skin. 



(8) The coccygeal vestige continues to grow after the 100 mm. stage. 



In the 67 mm. embryo, as can be seen in figure 46, the ventriculus terminalis 

 occupies a more cranial position than in the younger specimens; the conus medul- 

 laris has become relatively more slender and the filum terminale longer, the latter 

 disappearing caudal to the thirty-second vertebra, two remnants of the neural tube 

 being left. The membranes of the spinal cord are here also considerably further 

 developed than in the younger specimens. At this stage the arachnoid mem- 

 brane Ues between the dura mater and pia mater. At the upper border of the 

 twenty-seventh vertebra the dura mater leaves the wall of the vertebral canal 

 for the filum terminale, forming a sheath and reaching the filum terminale at a 

 level between the twenty-eighth and twenty-ninth vertebra?. In younger speci- 

 mens, for example in the 46 nun. embryo, this separation occurs at a level with 

 the thirty-fii-st vertebra. Therefore, the caudal end of the dural sac, as well as 

 the spinal cord, recedes cranialward. This phenomenon is an evidence of the 

 relatively more rapid growth of the vertebral column. What, then, is the cause 

 of the lengthening of the filum terminale? In the 33 mm. embryo I could recog- 

 nize distinctly, below the conus meduUaris, a bundle of fibers representing a primi- 

 tive filum terminale. In the 37, 39, and 50 mm. embryos this reaches almost to 

 the caudal end of the neural tube or the coccygeal medullary vestige. In the 37 

 and 39 nun. specimens it extends farther caudalward than in any of the others 

 (fig. 44) . It is my opinion, therefore, that the filum terminale consists at an early 



