198 DEVELOPMENT AND REDUCTION OF THE TAIL. 



Plate 4. 



Fig. 43. Embryo \o. 7.5, 30 mm. crown-rump length, enlarged 22. .5 diameters. The relative thinning-out of the 

 walls of the conus medu,llaris results in a tendency to their being thrown into large irregular folds. In 

 the re^^ion of the conus medullaris the regressive tendency sets in after that region has attained propor- 

 tions larger than those seen in the more caudal part. It consequently becomes expressed by a thinness 

 of the walls, producing a transparent terminal ventricle in contrast to the obliteration seen in the filum 

 terminate. 



Fig. 44. Embryo No. 972, 37 mm. crown-rump length, enlarged 18 diameters. This specimen shows the transition 

 of the atrojjhic sjjinal cord into a fibrous filum terminale. The terminal portion retains its lumen and 

 persists as the coccygeal medullary vestige. 



Fig. 4.T. Embryo No. 448, 52 mm. crown-rump length, enlarged 13.5 diameters. At this time the terminal ventricle, 

 the filum terminale, and the coccygeal medullary vestige are distinctly marked off from each other, and 

 their general adult characteristics attained. 



Fig. 46. Embryo No. 1656, 67 mm. crown-rump length, enlarged 9 diameters. The regressive condition of the walls 

 of the terminal ventricle are expressed by their relative thinness and their irregularity. The filum termi- 

 nale is almost entirely converted into a solid fibrous strand in which traces of ependymal masses can be 

 found. The membranes of cord can be seen and present an arrangement that closely simulates that of 

 the adult. 



