DEVELOPMENT OF THE NERVOUS SYSTEM 



823 



called the anterior, middle and posterior cerebral vesicles. These be- 

 come more and more distinct as development advances. The formation 

 of these parts is shown in Fig. 233. This figure, in C, shows the pro- 

 jections, on either side, of the vesicles that are eventually developed 

 (<?, Fig. 233, C) into the nervous portions of the organ of vision. 



The three cerebral vesicles now undergo further changes. The 

 superior, or the first primitive vesicle, is soon divided into two secondary 

 vesicles, the anterior of which becomes the cerebral hemispheres, and 

 the posterior, the diencephalon, including the optic thalami and the 

 third ventricle, which are eventually covered by the greater relative 

 development of the hemi- ABC 



spheres. The middle, or sec- 

 ond primitive vesicle, does 

 not undergo division and is 

 developed into the tubercula 

 quadrigemina. The poste- 

 rior, or third primitive vesicle, 

 is divided into two secondary 

 vesicles, the anterior of which 

 becomes the cerebellum, and 

 the posterior, which is cov- 

 ered by the anterior, the bulb 

 and the pons Varolii. While 

 this division of the primitive 

 cerebral vesicles is going 

 on, the entire chain of en- 

 cephalic ganglia becomes 

 curved from behind forward, 

 forming three prominent 

 angles. The first of these angles or prominences (e, Fig. 234, A, B y C\ 

 counting from before backward, is formed by a projection of the tuber- 

 cula quadrigemina, which at this time constitute the most projecting 

 portion of the encephalic mass; the second prominence (c t Fig. 234), 

 situated behind the tubercula quadrigemina, is formed by the projection 

 of the cerebellum ; the third (d t Fig. 234, A, B, C) is the bend of the 

 superior portion of the spinal cord. These projections and the early 

 formation of certain parts of the encephalon in the human subject are 

 illustrated in Fig. 234 (see also Plate XVI, Figs. 5 and 6). 



The cerebrum is developed from the anterior division of the first 

 primitive cerebral vesicle. The development of this part is more rapid 

 in its lateral portions than in the median line, which divides the cerebrum 

 imperfectly into two lateral halves, forming in this way the great longi- 



Fig.233. 



Development of the nervous system of the chick 

 (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 

 somites; b, posterior dilatation (the lumbar enlargement) ; 

 d, anterior dilatation of the neural canal ; i, 2, 3, anterior, 

 middle and inferior cerebral vesicles; a, slight flattening 

 of the anterior cerebral vesicle ; o, formation of the ocular 

 vesicles. 



