DEVELOPMENT OF THE NEKVOUS SYSTEM. 399 



tremities, 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 illustrated in Fig. 42, taken from Wagner, and 

 made more distinct by Longet, as they are drawn upon a 



Development of the nervous system of the chick. 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; &, posterior dilatation (the lumbar enlargement); 1, 2, 8, ante- 

 rior, middle, and inferior cerebral vesicles ; a, slight flattening of the anterior cerebral vesi- 

 cle; o, formation of the ocular vesicles. (LONGET, Traite de physiologie. Paris, 1869, tome 

 iii., p. 889.) 



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. 



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 hemispheres, and the posterior, the optic thalami, 

 which are eventually covered, by the greater relative develop- 

 156 



