174 DEVELOPMENT AND REDUCTION OF THE TAIL 



end (if the chorda dorsahs projects from the extreme end of the vertebral column into the 

 region of the non-vertebrated tail and appears as if iii'e\'ioiisly it may have adhered to the 

 wall of the caudal end of the neural tube. In the thirty-fourth vertebra, and lietween 

 the thirty-fourth and thirty-third, the chorda shows a typical loop formation, while in its 

 main portion it is almost straight and is situated in the midline of the column. The cen- 

 tral canal of the spinal cord narrows sharply at the level of the thirty-second vertebra. 

 Its ventral wall shows a few folds in the region of the atrophic portion of the cord. The 

 spinal cord reaches to the tip of the tail and has a continuous canal thoughout. The 

 middle sacral artery and vein extend into the non-vertebrated portion of the tail. 

 Embryo No. 453, 23 mm. Crown-Rump Length. 

 In embryo No. 453 there are 35 vertebrae, the last consisting of precartilage tissue 

 which has not as yet developed into true cartilage. The chorda dorsalis is straight and 

 runs through the vertebral column in the midline. Its caudal end, however, shows 4 

 coils, as shown in figure 40, representing a profile reconstruction of the specimen. The 

 neural canal narrows at the thirty-first vertebra. At the level of the thirty-third vertebra 

 a sac-shaped cell-mass lies between the spinal cord and the vertebral column, separated, 

 however, from the wall of the former (fig. 40, diverticulum). This sac seems to have 

 resulted from a diverticulum of the ventral wall of the neural canal, the stalk of which has 

 been obliterated. The \entral wall of the atrophic portion of the spinal cord shows small 

 folds at the level f)f the thirty-fourth and thirty-fifth vertebra^. The caudal end of the 

 spinal cord expands slightly and its extreme end enters into the tail, which is now quite 

 reduced. The midtlle sacral artery conmiunicates with the anterior spinal artery by 

 means of a branch that curves about the tip of the vertebral column. The subcaudal 

 epidermal plate has nearly disappeared, while the post-anal swelling and the coccygeal 

 tubercle have become visible. There is a shallow furrow between the tail-end and the 

 primordium of the coccygeal tubercle, constituting a boundary between them. This 

 embryo has 32 spinal ganglia with nerves. 



Embryo No. 382, 23 mm. Crown-Rump Length. 

 Embryo No. 382 has 34 cartilaginous vertebra?. The last three do not lie exactly in 

 a row in the median line, as can be seen in figures 15 and 4L The ventral portions of the 

 thirty-second and thirty-fourth vertebrae, and the dorsal portion of the thirty-third, have 

 become converted into cartilage, whereas the remainder of these two vertebrae still con- 

 sists of precartilage tissue, as in younger specimens. At the thirty-third vertebra the 

 chorda dorsalis gives off a short branch in a dorsal direction. Caudal to this the chorda 

 winds and finally disappears in the caudo-dorsal portion of the thirty-fourth vertebra. 

 Opposite the end of the chorda the wall of the spinal cord is so thickened as to give the 

 impression that the two might have been attached. It is to be regretted that in most of 

 the specimens the caudal end of the chorda dorsalis is torn. At the caudal end of the 

 last vertebra the middle sacral artery anastomoses with the anterior spinal artery through 

 a branch, similar to that mentioned in the last specimen. At the caudal end of the em- 

 bryo there is a bud-like structure of the skin. This proves to be a stunted tail, for at its 

 root can be recognized the sharp caudal end of the spinal cord and the terminal branches 

 of the middle sacral artery and vein. The central canal of the spinal cord narrows at the 

 thirty-second vertebra-, but the ventral wall of its atrophic portion does not exhibit the 

 folding that usually occurs in this region. 



Embryo No. 632, 24 mm. Crown-Rump Length. 

 Embryo No. 632 is quite similar to No. 382, just described, except that it has no tail- 

 bud. There are 35 vertebra^ and 31 spinal ganglia. The caudal end of the spinal cord 

 is represented by a strand of loosely arranged cells which extends to the epidermis at a 

 point corresponding approximately to the root of the tail in a younger embryo, as can be 

 seen in figure 16. 



