176 DEVELOPMENT AND REDUCTION OF THE TAIL 



Embryo No. 875, 27 mm. C"rown-Rump Length. 



In embryo No. 875 there are 34 vertebrae. The last is small and contains the wind- 

 ing part of the chorda dorsalis. The spinal cord narrows between the thirty-first and 

 thirty-second vertebrge, as shown in figure 20. Its caudal end expands sUghtly and the 

 extreme tip enters into the tail-bud. On the ventral wall of the central canal there are a 

 few small folds. Near the caudal end of the ^•ertebl•al column is a long, solid strand of 

 cells, similar in structure to the cells of the spinal cord, which may have become separated 

 from the latter at an earlier stage. Dorsal to the thirty-third and thirty-fourth vertebrae 

 is a small papilliform tail, which is non-vertebrated and contains the caudal end of the 

 vessels and a group of cells representing a remnant of the caudal end of the spinal cord. 

 There are 31 spinal ganglia with nerve-fibers. The coccygeal tubercle and post-anal 

 swelling are distinctly evident. 



Embryo No. 7.5, 30 mm. Crown-Rump Length. 



At the caudal end of embryo No. 75 there is a small papilliform tail containing a group 

 of cells which merge into the wall of the spinal canal, as shown in figure 43. The spinal 

 cord narrows suddenly at the mid-level of the thirty-second vertebra, and its atrophic por- 

 tion is further constricted at a level between the thirty-third and thirty-fourth vertebrae, 

 as indicated in figure 43 {constrict). The part below this constriction is the primordium of 

 the coccygeal medullary vestige and the upper part is destined in a later stage to undergo 

 retrogression, leaving a small cell-sac as a second coccygeal medullary vestige. There 

 are two large folds on the ventral wall of the spinal cord at a level with the thirty-first 

 vertebra. In the median plane they are triangular in shape and consist of ependymal 

 and mesenchymal cells that have been inverted, together with the wall. A large divertic- 

 ulum lies between these two folds. The space below the folds probably represents the 

 primordium of the ventriculus terminalis. Only the branches of the anterior spinal artery 

 enter into these folds. There are 34 cartilaginous vertebrae, and at thirty-first and thirty- 

 second vertebra? the column presents a typical curve. The chorda dorsalis shows a spindle- 

 shaped swelling between the vertebrae, and is much convoluted at the caudal end, as 

 seen in figure 43. There are 31 spinal gangUa; the nerves of the last pair are quite slender. 



Embryo No. 145, 33 mm. Crown-Rump Length. 



Embryo No. 145 has 35 vertebrae, as diagrammatically shown in figure 22. The 

 last one is situated on the dorsal side of the axis of the colunm, while the thirty-third and 

 thirty-fourth lean towards the ventral side. There are 31 spinal ganglia, the thirty-second 

 pair of nerves having no ganglia. In the caudal region there is a peculiarly shaped rem- 

 nant of the neural tube, possibly an anomaly of development, which is connected with the 

 main cord by a cell-strand. This cell-strand is directly continuous with the ependymal 

 layer of the primordium of the ventriculus terminalis, and may possibly be regarded as 

 the filum terminale. It emerges from the membranous sheath of the spinal cord, the more 

 cranial portion branching irregularly, while the caudal portion bends dorsally to enter 

 the minute tail-bud. The ends of the middle sacral artery and vein enter into the root 

 of the tail. In this embryo no coccygeal tubercle can be seen. 



Embryo No. 211, 33 mm. Crown-Rump Length. 



Embryo No. 21 1 has 34 vertebrae and 31 spinal ganglia. The vertebral column curves 

 ventrally at the thirty-first and thirty-second vertebrae. The caudal end of the chorda 

 dorsalis is undergoing regression and appears to be branching. The caudal end of the 

 spinal cord may be divided into three portions: (1) the primordium of the conus medul- 

 laris, which includes the primordium of the ventriculus terminalis; (2) the filum terminale; 

 (3) the coccygeal medullary vestige. The first extends about the length of the thirtieth 



