164 DEVELOPMENT AND REDUCTION OF THE TAIL 



2. In younger specimens (8 to 15 mm.) the tail appears as a free, pointed projec- 

 tion from the cloaca, directed caudo-dorsally. 



3. The tail consists of two portions — that containing vertebra; and that without ver- 

 tebrse. The latter contains only chorda dorsalis and medullary tube. In time this portion 

 disappears, the medullary tube atrophying and the chorda becoming converted into a knot. 



4. The vertebral portion persists for a while, appearing later as a coccygeal prominence 

 in the caudal region — coccygeal tubercle; then it, too, disappears. 



Keibel (1891) published in an important paper his findings in regard to the 

 development of the caudal gut in 4, 8, and 11.5 mm. embryos. The existence of 

 this structure, which forms a small canal or cell-strand at the caudal end of the 

 body axis, he regards as irrefutable evidence of a tail primordium. He found the 

 gut to be longer in the 8 mm. embryo than in the others. He asserts (page 378) 

 that in this stage the caudal gut extends through the whole length of the tail, and 

 apparently at this time attains its maximum length. Tliis author defines the 

 line of demarcation between the tail and the body in two ways: (1) he designates 

 as the tail the caudal portion beyond the attachment of the pelvic joint; (2) in the 

 younger embryos, in which the primordia of the legs have not yet appeared, he 

 defines the first 8 trunk segments as the cervical segments, the next 12 as the 

 dorsal, the next 5 as the lumbar, the next 5 as the sacral, and the remaining seg- 

 ments as caudal vertebrae. These he found were usually 6 in number. The last 

 one he called the mesodermic remnant and regarded it as one segment, although it 

 was two or three times as long as those cranial to it. 



Braun (1882) published his observations on the development and reduction 

 of the embryonic tail among mammalia, having at his disposal a great number of 

 specimens. As a rule he found a caudal filament at the extreme end of the tail 

 in the mammals that he studied, and therefore beUeved this structure to be of 

 general occurrence, and probably true also of the human tail. On the other hand, 

 Ecker and His, who studied the same condition in human embryos, did not con- 

 sider the two exactly homologous. Braun classified the two portions of the tail 

 as internal and external, and subdivided the latter into vertebral and non-vertebral 

 tail, the caudal filament being part of the latter. Waldej-er (1896) takes excep- 

 tion to this division into internal and external tail, as he does not believe the former 

 is a tail. Rodenacker (1898) uses the terms canda aperta and cauda occulta instead 

 of internal and external tail. Unger and Brugsch (1903) give the following results 

 of their investigations: 



1 . In the reduction of the tail the caudal vertebrae fuse to form the last vertebra. 



2. The caudal filament represents the remnant of the tail-bud and contains a branch 

 of the middle sacral artery. 



3. In the reduction of the tail two processes are concerned: (a) the formation of the 

 caudal tubercle; (6) the formation of the coccygeal tubercle. The first is the reduced 

 tail; the second is formed by the bulging of the caudal end of the vertebral column. This 

 is due to the fact that the growth of the vertebra is more rapid than that of the skin and 

 spinal cord. 



4. The connective tissue contained in the caudal filament develops into the caudal 

 ligament. 



s 



