AND OF THE CAUDAL END OF THE SPINAL CORD. 183 



It would appear, therefore, that in his subdivision of the mammahan embryonic 

 tail Braun included in the caudal filament that portion which lay between it and 

 the vertebrje. JNIy own position on the subject is briefly this: Is the caudal fila- 

 ment, through all the stages of mammalian embryonic life, one and the same thing as 

 the non-vertebrated tail? That an intermediate portion exists between the two was 

 apparently not recognized by Braun, but in man it constitutes a most important 

 factor in the reduction of the tail vertebrse. After detailed investigation with the 

 material at hand, numbering about 40 embryos ranging from 4 to 50 mm. in 

 length, I was able to divide the caudal structure as follows: (1) the vertebrated 

 portion; (2) the mesodermic end portion. In somewhat older embryos the first 

 is subdivided into a proximal portion with persisting vertebrae, and a portion from 

 which the primitive vertebra? have disappeared (lost-vertebrae portion). This 

 point can better be understood by referring to the embryos themselves. 



In embryo No. 221, 7.5 mm. long, the tail contains 38 somites and a long 

 mesodermic remnant. The somites, which later develop into precartilaginous 

 vertebrae, are well defined by the presence of small blood capillaries between them. 

 On the dorsal surface of the tail the boundaries of the somites can be recognized 

 distinctly as transverse shallow grooves. In the last somite, which is in contact 

 with the mesodermic remnant, the boundary is not nearly so clear as in the others 

 and would probably have disappeared altogether in the retrogressive process had 

 the embryo hved. In this specimen the tail is entirely a vertebrated tail, as each 

 somite is capable of development into a vertebra. The long mesodermic remnant 

 at the caudal end, although separated by segmentation from the mesodermal sheet, 

 evidently would not have developed into precartilaginous tissue. This part I 

 have differentiated from the vertebrated tail as a mesodermic remnant, using the 

 term employed by Keibel. I was able to distinguish in this embryo, therefore, 

 two divisions of the tail — a long somitic portion, the vertebrated tail, and a short 

 mesodermic portion, the caudal end of which may be compared with the caudal 

 filament but not with the lost-vertebra? tail. In this stage the non-vertebrated 

 portion has not as yet developed — that is, the portion in which the somites or precar- 

 tilaginous vertebrae have disappeared. The last somite, however, shows signs of 

 disappearing, and after a time, therefore, the lost-vertebrae portion will appear in 

 place of the last somite. In other words, the reduction phenomenon has begun 

 in the last somite; this progresses in the tail from one somite to the other, each 

 losing its distinct boundaries, the blood capillaries fusing and disappearing. 



In somewhat older specimens (8 mm., fig. 33), the last somite is larger than 

 the iireceding one and evidently rejjresents the fusion of two pieces — the thirty- 

 eighth somite and the mesodermic remnant. At the 1 1 mm. stage the lost-vertebrae 

 portion of the tail becomes well developed (fig. 34). In the 12 and 15 mm. em- 

 bryos the boundaries of the thirty-sixth, thirty-seventh, and thirty-eighth somites 

 have become indistinct. In the 15.5 mm. specimen these three somites are con- 

 verted into a cord which extends to the end of the tail (fig. 36, s(r. cell). This 

 cord consists of embryonic cells wliich at an earher stage of development existed 

 in the somites as sclerotomes. In the median portion of the cell-strand are three 



