Difference between revisions of "Category:Carnegie Embryo 184"

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Embryo No. 184, 50 mm Crown-Rump Length

Embryo No. 184 has 34 vertebrae, the last one being the smallest, as is indicated in figure 28. At the thirty-first vertebra the column presents a ventral curve, bringing the thirty-second, thirty-third, and thirty-fourth vertebrae in about a horizontal row and at right angles with the main column. The chorda dorsalis is disappearing in the 29 upper vertebral bodies, but at the thirtieth and below there is no change from the earlier stages, except that the chorda is relatively more slender. Its caudal end is bent caudo-dorsally before terminating; from this point the caudal ligament takes its origin. The middle sadral artery at this stage is a relatively delicate vessel, running from the ventral to the dorsal side of the vertebral column, and curving about the apex of the thirty-fourth vertebra. Its branches are plexiform, and in their meshes are groups of cells resembling neuroblast cells. The caudal end of the spinal cord contains a large cavity representing the ventriculus terminalis at a more advanced stage of development. The upper end of this cavity connects with the central canal of the spinal cord; its lower end terminates in two horns, the dorsal one of which is a blind pouch; the ventral horn is united with the caudal remnant of the spinal cord by a strand of ependymal cells and many transverse folds. The caudal remnant of the spinal cord consists of three separated portions. The first, which is attached to the caudal end of the ventriculus terminalis by an ependymal cell-strand, lies between the thirtieth and thirty-first vertebrae. This portion is embedded in nervefibers. As in younger specimens, it incloses a narrow cavity interrupted about midway. The second portion of the remnant is situated between the thirty-first and thirty-second vertebrae and leans to the dorsal side of the filum terminale. It also contains a small lumen. The third and largest portion is situated at the level of the thirty-third vertebra; its cavity is larger than the others and its caudal end enters into the caudal ligament.

The pia mater envelops the spina.l cord and contains blood capillaries. It traverses the course of the filum terminale, completely inclosing it, and appears to reach the dorsal portion of the thirty-third vertebra, at which point the filum terminale ends. The dura mater also covers the spinal cord over the pia mater. At the caudal end of the conus medullaris, about the thirtieth vertebra, the dura mater adheres closely to the pia mater. At the dorsal side of the thirty-third vertebra the fibers of the dura mater merge with the fibers of the caudal ligament.

This embryo has 31 spinal ganglia on the right side and 30 on the left. The last ganglion on either side is very small, being in process of retrogression. The right thirtieth and thirty-first ganglia and the left thirtieth are not located between the vertebrae, but at the dorsal side of the upper vertebral bodies.