THE SPINAL CORD AND BRAIN. 73 



enters the dorsal zone of the medullary tube, and a centrifugal process which passes 

 downward to join the ventral root. Other ganglionic cells are converted into the 

 medullary sheaths of the nerve-fibers, and still others migrate along the nerve- 

 libers and give rise to the peripheral or sympathetic ganglia. 



The cells which are retained permanently in the medullary wall differentiate 

 themselves into two classes: first, the so-called spongioblasts, which become the neu- 

 roglia of the adult, and second, the neuroblasts (Fig. 39, N}. Each medullary neuro- 

 blast produces a single neuraxon and several dendrites, although a small number 

 in certain positions remain always without dendrites. Most medullary neuraxons 

 are distributed within the spinal cord or brain, but some of those, developed from 

 cells in the ventral zone only, leave the medullary wall to produce the ventral (Fig. 

 39, V.r] and lateral nerve-roots. Ventral roots constitute the third, sixth, and 

 twelfth cephalic nerves, and enter into the composition of all the spinal nerves. 

 Lateral roots form part of the fifth, seventh, ninth, and tenth nerves, and the 

 whole of the eleventh cephalic nerve, but probably take no part in the formation of 

 any true spinal nerve. 



The gangiionic crest of the chick is figured and described in Chapter V. The 

 ganglia, roots, and nerves of the pig embryo are figured and described in Chap- 

 ter VI. 



* 



The Spinal Cord and Brain. 



The medullary tube, after giving off the cells which form the neural crest, 

 becomes the definite anlage of the spinal cord and brain. The differentiation of 

 the brain begins very early, and is marked by an enlargement of the medullary 

 tube in the region of the future head (compare Fig. 131). As seen there, the 

 brain takes up nearly half the entire length of the medullary tube. The widening 

 of the anterior portion of the medullary canal is not uniform, but tripartite. The 

 brain is divided by two narrower parts into three wider divisions, which are 

 termed the primary cerebral vesicles, and are named in their order fore-brain, mid- 

 brain, and hind-brain. The fore-brain widens very rapidly so ,as to form two lateral 

 projections, the optic vesicles (Fig. 131, op.V}. The division between the mid-brain 

 and the hind-brain is at first indistinct, but soon becomes sharply marked off. The 

 hind-brain then appears about as long as the other two vesicles combined, and 

 tapers down toward the spinal cord, into which it merges without demarcation. 



The walls of the medullary tube acquire throughout certain fundamental char- 

 acteristics, which are best studied in transverse sections. The side walls become 

 thickened, but the median, ventral, and median dorsal portions remain thin (Figs. 

 157, 158, Sp.c): The upper thin part is called the deck-plate; the lower thin part, 

 the floor-plate. Soon each lateral wall is divided into two longitudinal bands, 

 designated, respectively, the dorsal and the ventral zone (Fig. 116, D.Z, V.Z}. In 

 young embryos, as shown in the figure just cited, the two zones are. separated 

 from one another by an internal notch. Their morphological characteristics depend 



