188 DEVELOPMENT AND REDUCTION OF THiE tAlL 



CHORDA DORSALIS. 



In the 4 mm. embryo the chorda dorsahs hes close to the ventral side of the 

 neural tube, but cranial to the twenty-first segment it is separated from the tube 

 by the tissue of the primitive vertebrae. At this stage it forms a long, narrow tube, 

 its caudal end consisting of only a small cell-strand which terminates in a cell-mass 

 above the caudal extremities of the neural tube and caudal gut. In the 5 mm. 

 embryo, which is shown in figure 31, the chorda cranial to the thirty-third somite 

 is separated from the ventral side of the neural tube, while caudal to the thirty- 

 third the two are contiguous. The chorda terminates in the mesodermic remnant, 

 being covered by the ventral wall of the neural tube, and at its caudal end is united 

 with that of the caudal gut by a cell-strand. In specimens 7.5 to 11 mm. the 

 greater part of the chorda dorsahs cranial to the thirty-fourth or thirty-fifth somite 

 is embedded in the primitive vertebral column and shows considerable winding. 

 Caudal to the thirty-first somite the chorda is placed between the neural canal 

 and the primitive vertebral column. In passing down through the column it shows 

 a series of segmental undulating curves — that is, it alternately bends ventrally and 

 dorsally. The dorsal bends occur at the foci of vertebral formation which eventu- 

 ally become the intervertebral spaces. In older embryos— 12 to 14 mm. — this 

 segmental undulation of the chorda gradually disappears. In the 12 mm. embryo 

 (as shown in figure 35) the chorda is more completely embedded in the column, 

 although here its terminal portion emerges to he in the space between the spinal 

 cord and the tissue of the column. As the embryo advances in age this bending of 

 the chorda gradually decreases, until at about the 18 mm. stage it becomes straight 

 in its main portion, while the caudal part, which was hitherto straight, now becomes 

 curved, the first indication of the formation of undulations (compare figs. 35, 36, 37, 

 and 39) , which, however, are probably not segmental like those above described, but 

 are a phenomenon of the process of reduction in the caudal primitive vertebra?. 



When we compare the 7.5, 8, and 11 mm. embryos with those from 15 to 19 

 mm. it is easy to see that at first the few caudally situated primitive vertebrae— 

 the scleromeres — fuse together, and the chorda which is within them becomes 

 convoluted and recedes cranialward. The winding portion of the chorda is, there- 

 fore, situated in the last vertebra which has developed by the fusion of several 

 vertebrae. This condition remains the same up to the 18 mm. or even more ad- 

 vanced stage, and finally, in the 23 nmi. stage the chorda presents a spiral appear- 

 ance, as shown in figure 40. Braun found this same process in sheep and other 

 mammaUan embryos, the end of the chorda projecting caudalward from the last 

 vertebra. Ecker also noted this projection of the chorda in his mammalian mate- 

 rial. These authors recognized the winding or branched end of the chorda in the 

 caudal filament or in the extreme end of the tail and concluded that this was its 

 primitive state. In human embryos, however, as mentioned above, at the earliest 

 stage when the chorda reaches the extreme end of the tail, its caudal end is straight 

 and shows no winding until the reduction of the tail begins. In embryos from 15 

 to 23 mm. the caudal end projects almost caudalward from the last vertebra which 

 has been formed by the fusion of several vertebra?. His did not find such a condi- 



