AND OP THE CAUDAL END OF THE SPINAL CORD. 175 



Embryo No. 584«, 25 mm. Crown-Rump Length. 



A profile reconstruction of the caudal end of embryo No. 584a is shown in figure 42 

 and a simplified sketch is shown in figure 17. There are 34 vertebrae, the last being larger 

 than the thirty-third. Around the cartilaginous mid-portion of the last vertebra there 

 is considerable precartilaginous tissue, which has been formed by the fusion of several 

 scleromeres. At the thirty-first and thirty-second vertebrae the column curves ventrally. 

 The chorda dorsalis makes a loop in the thirty-fourth vertebra and gives off short branches. 

 The central canal narrows sharply at the level of the thirty-second vertebra, but expands 

 again at the dorsal portion of the thirty-third and thirty-fourth. This atrophic portion, 

 however, is not the primordium of the ventriculus terminalis. Unger and Brugsch com- 

 pare it with the sinus terminalis found in the amphibian embryo. It is my belief that it 

 represents the primordium of the coccygeal medullary vestige. The extremity of the 

 atrophic portion extends into the tip of the tail and is provided with a lumen throughout. 

 The caudal end of the chorda appears to have adhered to the ventral wall of the spinal 

 cord. There are 32 spinal ganglia, the thirty-second having slender nerves. The middle 

 sacral artery and \'ein enter into the tail, anastomosing through branches with the anterior 

 spinal arterj'. 



Embryo No. 405, 26 mm. Crown-Rump Length. 



Embryo No 405 has 34 vertebrae, as indicated in figure 18. The last one inclines 

 dorsally from the axis of the vertebral column. Between the thirtieth and thirty-first 

 vertebrae the axis of the ventral column shows a decided angle and below this point bends 

 ventrally, presenting the coccygeal curve which is characteristic of the adult. The chorda 

 dorsalis presents a spindle-shaped swelling at each intervertebral space and its caudal end 

 shows a loop-formation in the thirty-third vertebra. The spinal cord narrows at a level 

 between the thirtieth and thirty-first vertebrae; the atrophic portion, with its narrow 

 canal, is spiral at its caudal end and enters into the blunt tail-bud. On the dorsal surface 

 of this spiral part of the cord the epidermis is lacking; whether this is due to mechanical 

 injury or is a natural phenomenon could not be determined. From the ventral side two 

 branches of the anterior spinal artery enter. This embryo has 31 spinal ganglia com- 

 pletely supplied with nerves. The middle sacral artery anastomoses through a branch 

 with the anterior spinal artery. 



Embryo No. 1008, 26 mm. Crown-Rump Length. 



Embrj'o No. 1008 has 34 vertebrae, the last being larger than any of the others and 

 divided incompletely into two segments, as diagranmiatically shown in figure- 19. The 

 end of the chorda dorsalis presents a number of intricate coils and its caudal extremity lies 

 against the thick wall of the spinal cord. The spinal cord narrows at the thirty-second 

 vertebra, thus marking a boundary between the atrophic portion and the upper part of 

 the cord. The walls of the atrophic portion show two folds — one on the ventral, the 

 other on the dorsal wall. The former lies in the region of the thirty-fourth vertebra and 

 is similar to those already seen. The one on the dorsal wall, at a level between the thirty- 

 second and thirty-third vertebrae, is a diverticulum which projects caudo-dorsally and con- 

 tains a long, slender cavity continuous with the central canal. Such folds or diverticula 

 are seldom seen on the dorsal wall of the caudal end of the spinal cord. In the other 

 specimens studied folds were frequently encountered at this end of the cord, but always on 

 the ventral wall. On the ventral wall of the upper, wider part of the canal, at a level between 

 the twenty-ninth and thirtieth vertebrae, is another and much larger fold, extending down 

 about the length of two vertebrae. Both of its margins fuse with the ventral wall, thereby 

 forming a channel in the midUne of the fold which unites with the central canal. In this 

 specimen the spinal ganglia are 31 in number. 



