918 GENERATION. 



third (d, Fig. 300, 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. 300. 



The cerebrum, as we have just seen, 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 longitudinal fissure. At the same time, by the 

 rapid development of the posterior portion, it extends over the optic thalami, the cor- 

 pora quadrigemina, and the cerebellum. Up to the end of the fourth month, the hemi- 

 spheres are smooth on their surface ; but they then begin to present large depressions, 

 following folds of the pia mater, which .are the first convolutions, these increasing rap- 



FIG. 300. Development of the spinal cord and brain of the human subject. (Longet.) 



A, brain and spinal cord of an embryon of seven weeks ; lateral view. 



B, the same, from an embryon farther advanced in development; , spinal cord; d, enlargement of the spinal cord 



with its anterior curvature ; c, cerebellum ; e, tubercula quadrigemina ; /, optic thalamus ; #, cerebral hemi- 

 spheres. 



C, brain and spinal cord of an embryon of eleven weeks ; 6. spinal cord ; d, enlargement of the spinal cord, with 



its anterior curvature ; c, cerebellum ; e, tubercula quadrigemina ; gr, cerebral hemispheres ; o, optic nerve of 

 the left side. 



C', the same parts in a vertical section in the median line from before backward ; 5, membrane of the spinal cord 

 turned backward ; d, second curvature of the upper portion of the spinal cord, which has become thickened and 

 constitutes the peduncles of the cerebrum ; c, tubercula quadrigemina ; /J optic thalami covered by the hemi- 

 spheres. 



idly in number and complexity, especially after the seventh month. The septum lucidum 

 is then formed by an elevation of nervous matter from the base, which divides the lower 

 portion of the space left between the hemispheres as they ascend, and forms the two 

 lateral ventricles. At the base of these, are developed the corpora striata. The septum 

 lucidum is formed of two laminae, with a small space between them, which is the cavity 

 of the fifth ventricle. The posterior division of this first primitive vesicle forms the 

 optic thalami. These become separated in front into two lateral halves, but they remain 

 connected together at their posterior portion, which becomes the posterior commissure. 

 The central canal of the cord is prolonged upward between the optic thalami, and forms 

 the third ventricle, which is covered by the hemispheres. 



The second, or middle cerebral vesicle becomes filled with medullary substance, ex- 

 tends upward, and forms the peduncles of the cerebrum, the upper portion being divided 

 to form the tubercula quadrigemina. 



The anterior portion of the third primitive vesicle is developed into the cerebellum, 

 the convolutions of which appear at about the fifth month. Its posterior portion forms 

 the medulla oblongata, in the substance of which is the fourth ventricle, communicating 

 with the third ventricle by a little canal, the aqueduct of Sylvius, which is left in the 

 development of the middle vesicle. At about the fourth month, there is a deposition of 

 nervous matter in front and above, forming the pons Yarolii. 



In Fig. 299 (C, 0), it is seen that the vesicles for the organs of vision appear very early, 

 as lateral offshoots of the anterior cerebral vesicle. These gradually increase in size and 

 advance anteriorly, as development of the other parts progresses. We shall see, when 

 we come to study the development of the face, that the eyes are situated at first at the 

 sides of the head, gradually approaching the anterior portion. At the extremity of each 

 of these lateral prolongations, a rounded mass appears, which becomes the globe of the 



