Category:Carnegie Embryo 95

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Embryo No. 95, 50 mm Crown-Rump Length

Although embryo No. 95 is recorded in the catalogue of the Carnegie Collection as 40 mm. crown-rump length, its state of development more nearly corresponds with a 50 mm. embryo, and on this account I have used the latter measurement in the heading. This specimen has 35 vertebrae. The last one is very small and partly fused with the one above it. The column presents a ventral bend at the thirty-first vertebra, giving the typical coccygeal curve. The chorda dorsalis is disappearing in certain areas in the vertebral bodies as far down as the thirtieth vertebra, but in each intervertebral space a fragment remains. Caudal to the thirtieth vertebra the condition of the chorda remains the same as in the younger specimens, and in the thirty-second it gives off a short dorsal branch. The caudal end is more simple in form than in the younger stages, but I am inclined to believe that at an earlier stage it too was winding, as one can see in the thirty-fifth vertebra a few detached globules which probably at an earlier stage were continuous with the chorda and with it formed a terminal loop.

At the caudal end of the spinal cord are two groups of cells connected by a cell-strand. The more caudal one is situated dorsal to the thirty-fourth and thirty-fifth vertebra*; it is somewhat larger than the other, is oblong in form and incloses an oval cavity - a fragment of the central canal of the spinal cord. The other group of cells is situated dorsal to the thirty-second and thirty-third vertebra* and incloses a long, narrow cavity. The ventriculus terminalis extends the length of two vertebrae - the twenty-ninth and thirtieth. At this stage it has acquired its adult form. In none of the earlier specimens have I noted it so perfectly developed, although embryos No. 449, 30 mm., and No. 199, 35 mm., show a cavity at the caudal end of the central canal as the ])rimordium of the ventriculus. In this specimen the structure is cylindrical in shape, has six walls, and measures 0.87 mm long, 0.23 mm. deep, and 0.52 mm. wide. The ventral wall is concave, the dorsal convex, the sides slightly concave. The upper wall or ceiling is irregular and at the front presents a long, narrow diverticulum directed cranio-ventral. Behind this diverticulum is a narrow channel which connects the ventriculus terminalis and the central canal of the spinal cord. The ventriculus terminalis is embedded in the nerve-fibers of the cord. The filum terminale extends from the caudal end of the conus meduUaris, at the level of the thirty-first vertebra, to a point between the thirly-third and thirty-fourth vertebrae close to the column. It is covered by a membrane of the spinal cord and passes through the ventral side of the cell groups at the caudal end of the medullary tube. The pia mater covers closely the whole surface of the spinal cord; it contains blood capillaries, and is visible at the conus meduUaris. The dura mater, which envelops loosely the pia mater, adheres to the wall of the vertebral canal as far as the midlevel of the thirty-first vertebra, at which point it leaves the wall and unites with the caudal end of the conus medullaris. This portion constitutes the primordium of the bursa durge matris. After the dura mater reaches the conus medullaris it envelops the pia mater quite closely, both following a caudal course and forming a sheath for the filum terminale. The point at which these membranes terminate can not be definitely decided. It is probable that the pia mater extends nearly to the end of the filum terminale between the thirty-third and thirty-fourth vertebrae. The fibers of the dura mater appear to enter into the caudal and dorsal portions of the last vertebra.

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