178 DEVELOPMENT AND REDUCTION OF THE TAIL 



Embryo No. 362, 39 mm. Crown-Rump Length. 



Embryo No. 362 has 34 vertebrae, the last one being situated on the dorsal side of the 

 vertebral axis, as is diagrammatically shown in figure 26. At the thirtieth and thirty- 

 first vertebra? the column is bent ventrally. The caudal end of the chorda dorsalis winds 

 and branches in the last three vertebrae. There are 30 spinal ganglia with nerves, but the 

 thirty-first pair of nerves has no ganglia, their degeneration probably having occurred 

 before that of the nerves. The caudal portion of the spinal cord is divided into the conus 

 medullaris, filum terminale, and coccygeal medullary vestige. The ventriculus terminalis, 

 which is included in the conus medullaris owing to the folding of its walls, is subdivided 

 into two parts — an upper part, triangular in shape, and a lower, which is oblong and com- 

 municates with the upper by a narrow channel. The filum terminale reaches from the 

 caudal end of the conus medullaris to the ventral side of the coccygeal vestige, being 

 enveloped by the membrane of the spinal cord, the dura mater. This embryo presents a 

 small tail-bud at its caudal end, containing a group of cells which connects with the 

 coccygeal medullary vestige. 



Embryo No. 95, 50 mm. Crown-Rump Length. 



Although embryo No. 95 is recorded in the catalogue of the Carnegie Collection as 

 46 mm. crown-rump length, its state of de\'elopment 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 typi- 

 cal coccygeal curve. The chorda dorsahs 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 looj). 



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 vertebrae; 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 vertebrae and incloses a long, narrow ca\'ity. The ventric- 

 ulus 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, 36 mm., and No. 199, 35 mm., show 

 a cavity at the caudal end of the central canal as the primordium of the ventriculus. In 

 this specimen the structure is cylindrical in shape, has six walls, and mea.sures 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 nar- 

 row channel which connects the ventriculus terminalis and the central canal of the s])inal 

 cord. The ^•entriculus terminalis is embedded in the nerve-fibers of the cord. The filum 

 terminale extends from the caudal end of the conus medullaris, at the level of the thirty- 

 first vertebra, to a point between the thirty-third and thirty-fourth vertebrae, close to the 

 column. It is covered by a membrane of the spinal cord ami 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 medullaris. The dura mater, which envelops loosely the pia mater, adheres 



