194 



DEVELOPMENT AND REDUCTION OF THE TAIL 



usually found a ganglion; another occurs approximately at a point between the 

 thirty-first and thirty-second vertebrae. From the latter ganglion the single nerve- 

 trunk follows the course of the middle sacral artery and vein, running between 

 them, and emerges dorsally from the caudal end of the ver- 

 tebral colunm, where the coccygeal medullary vestige and 

 caudal ligament curve about the apex of the column. This 

 nerve consists of a large bundle of non-medullated nerve- 

 fibers, but the structure of its caudal end can not at this 

 stage be made out distinctly. At a level between the 

 thirty-third and thirty-fourth vertebrae, or perhaps a little 

 above, there is a small group of cells representing a sym- 

 jiathetic ganglion. At this point are frequently found nu- 

 merous jjlexiform branches of blood-vessels enmeshing this 

 group of cells. This richly vascularized cell-group may be 

 the primordiiun of the glandula sacralis. 



-Vert 



-Ganq.symp- 



-Carl Intv 



Text-figure 2. 



Vpntr.ll view of caudal portion of 

 vertebral column, showing char- 

 acteristic arrangement of sym- 

 pathetic ganglion-strands in hu- 

 man embryos between 46 and 

 67 mm. long. Cart, inlv., inter- 

 vertebral fibrocartilage ; Gang, 

 si/mp., ganglionic node; Veil., 

 body of \ertebra. 



SUMMARY. 



(1) The human embryo possesses a true tail composed 

 of ]jrimitive vertebrae and the caudal ends of the spinal 

 cord, chorda dorsalis, and middle sacral artery and vein. 



(2) The longest and most completely developed tail 

 among the specimens examined by me was found in a 7.5 

 mm. embryo. This was 1.2 mm. in length. 



(3) The human embryo does not possess a caudal filament homologous with 

 that of other mammals. 



(4) The reduction of the tail, especially of the primitive vertebrae, begins when 

 the embryo has reached a length of about 8 or 9 mm. 



(5) Prior to this the tail consists of two parts: a proximal longer part (the 

 vertebrated tail), wliich has well-formed somites, and a caudal shorter part which 

 contains only a mesodermic remnant. 



(6) In embryos from 25 to 27 mm. the tail is reduced to a small papilla, in 

 which are contained the caudal ends of the spinal cord and the middle sacral artery 

 and vein, and into wliich the end of the caudal ligament enters. As a rule this 

 tail-bud is not situated directly at the caudal extremity of the vertebral column, 

 but slightly dorsal to it. The vertebrated portion of the external tail has retracted 

 into the soft tissues and has thus become an internal taU, whereas the lost-vertebrae 

 tail projects temporarily and finally it also disappears. 



(7) At the time the division of the external tail takes place two eminences 

 appear at the caudal region; one ventral (coccygeal tubercle or Steisshocker), the 

 other dorsal (caudal tubercle or Kaudalhocker). The first is due to the pushing 

 up of the caudal end of the internal-tail vertebrae, formerly situated in the root 

 of the external tail and constituting the vertebral portion of it in younger embryos. 

 The second is usually shaped like a small papilla and by some authors is called 

 the tail-bud or caudal filament, although the latter, as stated above, is entirely a 



