DEVELOPMENT OF THE NERVOUS SYSTEM. 



819 



at about the end of the second month, while the arachnoid is not distinct 

 until the fifth 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. 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 dilatations are observed at the superior 

 and at the inferior extremities of the neural canal. The cord is nearly uni- 

 form in size in the dorsal region, marked only by the regular enlargements 

 at the sites of origin of the spinal nerves ; but there soon appears 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 posterior cerebral vesicles. These become 

 more and more distinct as development advances. The formation of these 

 parts is shown in Fig. 303. This figure, in C, shows the projections, on either 

 side, of the vesicles which are 

 eventually developed (0, Fig. 

 303, C) into the nervous por- 

 tions of the organ of vision. 



The three cerebral vesicles 

 now undergo farther changes. 

 The superior, or the first prim- 

 itive vesicle, is soon divided 

 into two secondary vesicles, the 

 anterior of which becomes the 

 cerebral hemispheres, and the 

 posterior, the optic thalami, 

 which are eventually covered 

 by the greater relative develop- 

 ment of the hemispheres. The 

 middle, or second primitive 

 vesicle, does not undergo divis- 

 ion and is developed into the 

 tubercula quadrigemina. The 

 posterior, or third primitive vesicle, is divided into two secondary vesicles, 

 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 (e, Fig, 304, 

 A, B, C), counting from before backward, is formed by a projection of the 

 tubercula quadrigemina, which at this time constitute the most projecting 



FIG. 303. Development of the nervous system of the chick 

 (Longet, after Wagner). 



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 

 protovertebree ; ft, posterior dilatation (the lumbar en- 

 largement) ; d, anterior dilatation of the neural canal ; 

 1, 2, 3, anterior, middle and inferior cerebral vesicles; a, 

 slight flattening of the anterior cerebral vesicle; o, for- 

 mation of the ocular vesicles. 



