Category:Carnegie Embryo 972

Embryo No. 972, 37 mm Crown-Rump Length

Embryo No. 972 has 34 vertebrae, the last two having fused together at the center. The vertebral column curves ventrally at the thirtieth and thirty-first vertebrae, the curve being so sharp that the thirty-second, thirty-third, and thirty-fourth vertebrae are situated in a row nearly horizontal to the trunk, as can be seen in figure 44. Cranial to thirty-first the chorda dorsalis expands between the vertebrae. The caudal end is winding and broken.

At the caudal end of the spinal cord one can recognize the primordia of the conus medullaris, filum terminale, and coccygeal medullary vestige, as shown in figure 44. The primordium of the ventriculus terminalis, which is included in the conus medullaris, appears as a continuation of the central canal of the spinal cord without any line of demarcation, and is situated at a level with the twenty-ninth vertebra. Its ventral wall is thinner than the dorsal wall and shows a few small folds (fig. 44, x). The primordium of the filum terminale extends from the caudal end of the conus medullaris, viz, at the level of the under part of the thirtieth vertebra, to the middle of the thirty-second vertebra. It contains an incomplete canal which is lined by a remnant strand of ependymal cells which are directly continuous with the ependyma of the ventriculus terminalis. At its caudal end is an ependymal strand which is directly continuous with the coccygeal medullary vestige. In addition to this ependymal substance, there is a small bundle of nerve-fibers along the ventral border of the filum terminale which extends into the white substance of the cord above. The primordium of the coccygeal medullary vestige is situated dorsal to the last two vertebra and contains a slender cavity. There are 30 spinal ganglia supplied with complete nerves. The thirty-first ganglion has almost completely disappeared on each side, leaving the nerves exposed.