HISTOLOGY. 



arbitrarily drawn. The relations of the medullary tube to other structures 

 in the embryo have been shown in Figs. 19-21, p. 19-22. 



sp-g 



FIG. 109. THE DEVELOPMENT OF THE NERVOUS SYSTEM AS SEEN IN CROSS SECTIONS OF RABBIT 



EMBRYOS: A, 7^ DAYS; B, 8i DAYS; C, 9 DAYS; D, 10$ DAYS; E, 14 DAYS. 



c. c., Central cavity; d. r., dorsal root; d. ra., dorsal ratnus; ep., ependymal layer; g. c., ganglion cells; 

 2.1., gray layer ; m. g., medullary groove ; m. t., medullary tube; o. b., oval bundle ; s. g., sympathetic 

 ganglion; sp. g., spinal ganglion; s. ra., sympathetic ramus; v. r., ventral root; v. ra., ventral ramus; 

 w. 1., white layer. 



THE SPINAL GANGLIA. At about the time when the medullary tube 

 separates from the epidermal ectoderm, some cells which are detached 

 from its median dorsal portion pass down on either side of the tube, as 

 shown in Fig. 109 C and D. Through mitotic division these cells ac- 

 cumulate in paired masses corresponding in number with the segments 

 of the body. They are the spinal ganglia. A typical cell of a spinal 



Bipolar cells. 



T-cell. 



ganglion is at first round, but later becomes 

 bipolar by sending out two processes, one 

 toward the periphery and the other toward 

 the medullary tube. These processes grow 

 out from opposite sides of the cell (Fig. no). 

 With further growth the nucleated cell body 

 passes to one side of its prolongations, with 

 which it remains connected by a slender stalk. 

 These T-shaped cells are characteristic of 

 the spinal ganglia. The fibers which grow toward the medullary tube 

 enter its outer part and fork, sending one branch toward the brain and the 

 other down the cord. There are many of these parallel fibers extending 

 toward the brain so that they form distinct bundles, one on either side of 

 the cord, known as oval bundles (Fig. 109, E). Since they receive acces- 



FIG. no. SPINAL GANGLION CELLS, 

 THB BIPOLAR FORMS FROM A 6 

 DAY CHICK EMBRYO. 



