72 



THE EARLY DEVELOPMENT OF MAMMALS. 



striction, and one also between the second and the third. The three vesicles are 

 the anlages, respectively, of the fore-brain, mid-brain, and hind-brain. 



In the region of the spinal cord the medullary tube soon becomes somewhat 

 flattened from side to side, and therefore acquires a characteristic oval configura- 

 tion as seen in cross-section (Fig. 38). We can now recognize in the cross-sections 

 four regions: first, the two thick sides; second, in the median dorsal line the thin 

 portion which we call the deck-plate, and in the median ventral line the thin por- 

 tion which we call the floor-plate. Later on, each lateral portion becomes sub- 

 divided into two longitudinal bands, known as the zones of His, and distinguished 

 from one another as the dorsal and ventral zones (Fig. 116, D.Z, V.Z}. After this 

 stage there are six longitudinal zones in the embryonic cord. These are, first, the 

 deck-plate; second and third, the dorsal zones of His; fourth and fifth, the ventral 

 zones of His; and sixth, the floor-plate. These six zones also appear in the region 

 of the brain, where, however, .they undergo characteristic modifications. The zones 



of His dominate the entire morphology 

 of the central nervous system, because 

 a ^ t ^ ie sensor y nerves enter the central 

 nervous system at the lower edge of the 

 dorsal zone and primarily ramify in the 

 dorsal zone, and, further, because all 

 efferent nerve-fibers arise in the ventral 

 zone and pass out to the body at certain 

 points on the surface of the ventral zone. 



Origin of Nerves. 



The essential constituents of a nerve 

 are the neuraxons, each neuraxon being 



FIG. 39. TRANSVERSE SECTION OF THE DORSAL CORD the prolongation of a nerve cell. In ad- 

 AND GANGLION OF A CHICK OF NINE DAYS. dition, the neuraxons are usually sur- 



V.r, Anterior root. Dr, Posterior root. Gl, Ganglion of roim ded by medullary sheaths. The 

 dorsal root, col, Collateral, with its branches. N, 



Medullary neuroblasts with dendrites and axis- y Un g Cells from whlch the neuraxons 

 cylinder. (The drawing is from a Golgi prepara- grow Out are called neuroblasts. The 

 tion. No sheath or neuroglia cells are repre- olfactory nCrve has a Special historv 

 sented.) (After Cajal.) 



(compare page 76). All the other 



nerves are produced by neuroblasts, which are developed from the cells of the 

 medullary tube. 



Shortly after the medullary groove has closed to form a tube, a number of 

 cells migrate from the medullary wall, just at the junction of the dorsal zone and 

 the deck-plate, and in sufficient numbers to form a longitudinal band, which is 

 known as the ganglionic crest, which soon, however, breaks up into separate masses, 

 the ganglia, which develop symmetrically. Some of the cells of each ganglion 

 become neuroblasts (Fig. 39, Gl}, each of which forms a centripetal process which 



