184 DEVELOPMENT AND REDUCTION OF THE TAIL 



or four segments, and the last vertebra also shows two or three divisions. At this 

 stage three types of vertebrae can be recognized at the caudal end of the vertebral 

 column: (1) the vertebrae which have developed from the sclerotomes into precar- 

 tilaginous or primitive vertebrae; (2) the incomplete primitive vertebrae, or the 

 parts of the thirty-sixth and thirty-seventh sclerotomes which form the last verte- 

 brae; (3) the cell-strand formed by the fusion of the last two somites and perhaps 

 the thirty-sixth as well. I have not been able to determine whether or not the 

 mesodermic remnant has merged into this strand. This mesodermic cell-strand — 

 the primordium of the caudal ligament — is diagrammatically shown in figures 36 

 and 37 (str. cell). In the 16, 17, and 18 mm. embryos the last vertebra is larger 

 than the more proximally situated ones and consists of two or three pieces united 

 in the median plane; 35 vertebrae, developed into precartilage or cartilage tissue, 

 were found in the 15.5, 17, and 18 iimi. embryos, and 36 in the 16 mm. embryos. 

 In those 17 mm. and larger the last two or three primitive vertebrae were usually 

 found to be fusing at the center of the column, while in the lateral parts they show 

 the divisions quite distinctly. 



His did not find any extra vertebrae in the tail, but many other authors have 

 recognized from 2 to 4. Perhaps the material upon which His based his studies 

 did not include specimens in the same stages of development as my 15.5, 17, and 18 

 mm. embryos, in which the last vertebra consisted of two or three pieces. The 21 

 mm. embryo also represents the typical condition at this stage, the tail showing 

 extra vertebrae. In this embryo can be clearly demonstrated a short tail consisting 

 of two portions, such as has been described by His, the extreme non-vertebrated 

 or, more correctly speaking, the lost- vertebrae portion, and the vertebrated por- 

 tion. I am sure that embryos of this age never present a caudal filament 

 homologous with that of other mammals, and I can not therefore agree with His, 

 Braun, Keibel, linger, and others, who describe the non-vertebrated portion of 

 the tail in the human embryo as a caudal filament, since this portion at an earUer 

 period contained somites capable of development into primitive vertebrae. Ecker, 

 who studied human and mammalian embryos, asserted that the human embryo 

 never has the caudal filament such as is the rule for mammalia. I could not recog- 

 nize clearly a caudal filament in any of my specimens. In one of the three embryos 

 (6.5, 7.5, and 8 mm.) which showed a portion of the caudal gut in the end of the 

 tail, the caudal end was demarcated by a bend, and this might have been mistaken 

 for a caudal filament. I am of the opinion that in the 8 and 10 mm. embryos the 

 portion of the tail beyond the pointed end of the vertebral column, as shown in 

 figiu-e 34, can be compared with the caudal filament in mammals, but is not the 

 true caudal filament described by Braun. It consists of the caudal end of the 

 mesodermic remnant and contains the end of the neural canal and, in the 8 mm. 

 embryo (fig. 33), a part of the caudal gut. I beheve, also, that the non-vertebrated 

 portion of the mammalian tail, which is not included in the caudal filament, is 

 homologous with the non-^'ertebrated tail of the human embryo. 



Having compared the non-vertebrated portion of the mammalian tail with 

 that of human embryos, I have concluded that in the former the reduction process 



