. Carnegie Institution of Washington publication. 194 LE DÉVELOPPEMENT ET RÉDUCTION DE LA QUEUE se trouve habituellement un ; un autre ganglion se produit environ à un point entre la trente et unième et trente-deuxième vertèbres. À partir de ce dernier le ganglion nerveux- tronc unique suit le cours de l'artère sacrale du milieu et d'idées, l'exécution entre eux, et émerge dorsalement à partir de l'extrémité caudale du ver- tebral colunm, où l'coccygien vestige médullaire et ligament caudal courbe autour du sommet de la colonne. Ce nerf est composé d'un grand paquet de nerf non- myéline des fibres, mais la structure de son ca

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. Carnegie Institution of Washington publication. 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-

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