DEVELOPMENT OF THE NERVOUS SYSTEM. 917 



month. The nerves are not produced as prolongations from the cord into the various 

 tissues, nor do they extend from the tissues to the cord ; but they are developed, in each 

 tissue, by a separation of histological elements from the cells of which the parts are 

 originally constituted, which at first appear to be identical in their morphological char- 

 acters. The nerves of the sympathetic system are developed in the same way. 



The mode of development of the spinal cord is thus sufficiently simple ; but, with the 

 growth of the embryon, we observe dilatations at the superior and at the inferior extrem- 

 ities of the neural canal. The cord is uniform in size in the dorsal region, marked only 

 by the regular enlargements at the sites of origin of the spinal nerves ; but we soon 

 observe an ovoid dilatation below, which forms the lumbar enlargement, from which the 

 nerves are given off to the inferior extremities, and the brachial enlargement above, where 

 the nerves of the superior extremities take their origin. At the same time, there is a 

 more marked dilatation of the canal at its cephalic extremity. Here, a single enlargement 

 appears, which is soon divided into three vesicles, called the anterior, middle, and poste- 

 rior cerebral vesicles. These become more and more distinct as development advances. 

 The formation of these parts is illustrated in Fig. 299, taken from "Wagner, and mado 

 more distinct by Longet, as they are drawn upon a black ground. This figure, in C, 

 shows the projections, on either side, of the vesicles which are eventually developed 

 into the nervous portions of the organ of vision. 



FIG. 299. Development of the nervous system of the chick (Longet ) 



A, the two primitive halves of the nervous system, twenty-four hours after incubation ; B, the same, thirty-six hours 

 after ; C. the same, at a more advanced stage, c, the two primitive halves of the vertebrae ; d, anterior dilata- 

 tion of the neural canal; A, posterior dilatation (the lumbar enlargement) ; 1, 2, 8, anterior, middle, and inferior 

 cerebral vesicles ; a, slight flattening of the anterior cerebral vesicle ; o, formation of the ocular vesicles. 



The three cerebral vesicles now undergo farther changes. The superior, which we 

 may call the first primitive vesicle, enumerating them from above downward, is soon 

 divided into two secondary vesicles, the anterior of which becomes the cerebral hemi- 

 spheres, and the posterior, the optic thalami, which are eventually covered, by the great- 

 er relative development of the hemispheres. The middle, or second primitive vesicle, 

 does not undergo division and is developed into the tubercula quadrigemina, or centres 

 of vision. The posterior, or third primitive vesicle, is divided into two secondary vesi- 

 cles, the anterior of which becomes the cerebellum, and the posterior, which is covered 

 by the anterior, the medulla oblongata and the pons Varolii. While this division of the 

 primitive cerebral vesicles is going on, the entire chain of encephalic ganglia becomes 

 curved from behind forward, forming three prominent angles. The first of these angles 

 or prominences (, Fig. 300, A, B, C), counting from before backward, is formed by a 

 projection of the tubercula quadrigemina, which, at this time, constitute the most pro- 

 jecting portion of the encephalic mass ; the second prominence (>, Fig. 300), situated 

 behind the tubercula quadrigemina, is formed by the projection of the cerebellum ; the 



