AND OF THE CAUDAL END OF THE SPINAL CORD. 189 



tion in human embr^yos, although it is usual. According to Braun, the occurrence 

 of a free, naked end of the chorda is due to the disappearance of the last primitive 

 vertebrae by which it had previously been surrounded. In the human embryo the 

 caudal end of the chorda was never surrounded by primitive vertebrae, but was 

 situated between the neural canal and the primitive vertebral column. In the 

 25 mm. (fig. 42) and the 30 mm. embryo (fig. 43) the coil-hke appearance of the 

 chorda is typical, and these stages, therefore, are the clearest of any thoughout 

 embryonic hfe. Later on, for example, in the 37 mm. embryo (fig. 44), the caudal 

 end of the chorda is disappearing, leaving a few remnants which have become 

 separated from the main chordal strand. This degenerative fragmentation and 

 partial absorption of the terminal portion of the chorda results in a great variety 

 of forms. Very seldom is the caudal end branched in the earlier stages. From the 

 39 mm. stage the chorda becomes gradually reduced and is finally converted into 

 a more simple form, as shown in figures 45 and 46. While the short caudal por- 

 tion shows the above-mentioned variations, the main strand changes but slightly. 

 After it becomes straightened some portions of it which lie in the intervertebral 

 fibro-cartilage show spindle-shaped swellings, as shown in figures 39 and 42 (18 

 to 25 mm.). At last, in the 50 and 52 mm. embryos, the parts embedded in the 

 vertebral bodies disappear, leaving small remnants in the intervertebral spaces. 

 Frequently these remnants show visible coils, as can be seen in figure 46. These 

 remain often until a later stage. The disappearance of the chorda below the thir- 

 tieth vertebra occurs later than that of the main strand, and we can therefore still 

 recognize it in the 67 mm. embryo as a continuous cord through the caudal vertebral 

 bodies. 



DEVELOPMENT OF THE CAUDAL END OF THE SPINAL CORD. 



In the 4 mm. embryo the caudal end of the neural tube fuses with the solid 

 mass of mesodermal cells which extends to the ventral side of the tail. The caudal 

 ends of the chorda dorsalis and caudal gut also merge with this cell-mass. In the 

 5.5 to 7.5 mm. embryos the caudal end of the neural tube, with its central canal, 

 extends to the apex of the tail and merges into the mesodermal cell-mass, entirely 

 losing its boundaries. In the 8 mm. specimen a difference can be plainly recog- 

 nized between the caudal portion of the spinal cord and the portion that lies cranial 

 to the thirty-second somite. Caudal to this level the central canal is distinctly 

 narrower. Thus it may be divided into two portions, an upper, wider canal and 

 a caudal narrow or atrophic canal. The former constitutes the main part of the 

 central canal of the spinal cord. On cross-section it is oval in shape and its walls 

 show no folds. The caudal part is narrower in its dorso-ventral diameter than is 

 the main canal, and therefore on section presents a more rounded form. Some- 

 times a large fold is found between the two parts and in the 11 and 12 mm. speci- 

 mens can be seen the primordia of other folds on the walls of the atrophic canal, 

 especially on the ventral side, as shown in figures 34 and 35. The distinction 

 between the atropine portion of the spinal cord and the main part is quite marked 

 in the 15.5, 16, 17, and 19 mm. embryos. The caudal end of the wider canal 

 expands transversely, and where it narrows into the atrophic canal constitutes the 



