AND OF THE CAUDAL END OF THE SPINAL CORD. 185 



occurs at an early stage, just as it does in the human embryo. This theory is 

 based upon two facts: First, the last caudal vertebra is larger than the next 

 proximally situated one, as in the cow embryo described by Braun, its increased 

 size being due to the fusion of the last three pieces. This phenomenon represents 

 the reduction process in the lower segments at a certain stage. Second, the num- 

 ber of vertebrae in the tail of sheep embryos, as asserted by Braun, is variable, and 

 this variation must be due to a stronger or weaker effect of reduction. 



In the 19 and 23 mm. specimens there is a very short tail with a caudal tubercle. 

 In the 19 mm. embryo the vertebrae of the tail have fused together into one large 

 vertebra — the thirty-third — in which can be recognized two or three pieces. The 

 caudal end of the chorda dorsahs within this vertebra shows several coils, indi- 

 cating a fusion of the last few vertebrae. The lost-vertebrae portion of the tail is 

 represented in this specimen by the caudal extremity which contains the ends of 

 the neural canal, the middle sacral artery and vein, and the chorda dorsalis. The 

 latter adheres to the ventral wall of the neural canal and it appears as if the neural 

 tube is retracted cranialward. In the 23 mm. embryo the development of the 

 caudal end is farther advanced. The furrow between the tail root and the primitive 

 anus becomes gradually more shallow, and the vertebral portion of the tail is em- 

 bedded in the embryonic tissue which will later develop into the coccygeal tubercle. 

 This shortening of the tail is evidently brought about by three factors: (1) fusion 

 of the last few caudal vertebrae; (2) rapid growth of the aUmentary canal and its sur- 

 rounding structures; (3) the flexion of the caudal portion of the vertebral column. 



(1) The disappearance of the last few caudal vertebrae by fusion, leaving only 

 the winding end of the chorda dorsahs, is a well-estabhshed proof of the compres- 

 sion and final disappearance of the caudal vertebrae and the chorda dorsalis which 

 was within them, linger and Brugsch took the view that in spite of the presence 

 of an external tail one could still speak of the formation of a coccygeal tubercle, 

 inasmuch as the segments of the caudal region, which in their most caudal portion 

 are already reduced, have begun to show a moderate, ventrally directed curve in 

 their axis, which is eventually to be the coccygeal eminence. They point out 

 that two factors are of importance in the formation of the coccygeal eminence: 

 (a) the fusion (reduction) of the most caudal segments; (b) the bending in the axis 

 of the caudal vertebrae. In the 25 and 27 mm. embryos the lost-vertebrae portion 

 of the tail becomes rounded off and is shown as the caudal tubercle. Its extremity 

 appears as a bud-shaped appendage and contains the caudal ends of the spinal 

 cord, with its central canal, and of the middle sacral artery and vein. This bud- 

 Uke appendage is called by many authors the caudal filament, but this is incorrect 

 for the reason, as stated above, that it represents only a part of the lost- vertebrae 

 portion of the tail which was primarily the vertebral portion, and therefore could 

 never be considered as the caudal filament described by Braun. 



(2) The area between the vertebral column and the rectum, especially the 

 root of the tail, increases rapidly in a caudo-ventral direction. The caudal region 

 of the rectum also extends down, its growth being in proportion to that of the 

 vertebral column (figs. 39, 40, and 43). It is the beUef of many authors— Rosen- 



