820 GENERATION. 



portion of the encephalic mass ; the second prominence (c, Fig. 304), situated 

 behind the tubercula quadrigemina, is formed by the projection of the cere- 

 bellum ; the third (d, Fig. 304, 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. 304. 



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 longitudinal fissure. At 



the same time, by the rapid 

 development of the posterior 

 portion, it extends over the 

 optic thalami, the corpora 

 quadrigemina and the cere- 

 bellum. Until the end of the 

 fourth month, the hemi- 



FIG. ^.-Development of the spinal cord and brair, of the S P her6S are Sm th On thelr 

 human subject (Longet, after Tiedemann). SUrf aC6 ; but they then begin 



A, brain and spinal cord of an embryon of seven weeks ; lat- , , , n 



eraiview. to present large depressions, 



B, the same, from an embryon farther advanced in develop- * ,-, * TJ XT, 



ment; 6, spinal cord; d, enlargement of the spinal cord, following tolds 01 the pia ma- 

 with its anterior curvature; c, cerebellum ; e, tubercula j.^, , T i i ^ <i ^ n 



quadrigemina ; /, optic thalamus ; g, cerebral hemi- ter, Wllicn are tne IirSt COn- 



C, braizf and spinal cord of an embryon of eleven weeks ; 6, Volutions, these increasing 



spinal cord ; d, enlargement of the spinal cord, with its ,, /n TT ,'-~ ,V*. >-^A 

 anterior curvature ; c, cerebellum ; e, tubercula quadri- rapidly in number and COm- 



feTside: "' cerebral hemispheres; ' optic nerve of the plexity, especially after the 



C', the same parts in a vertical section in the median line, from apvfm f Vi rm +V Th P OOTY!- n 



before backward ; ft, membrane of the spinal cord, turned se veiltn montn. 1 ne Septum 



backward; d, second curvature of the upper portion of In pi /him ifhpn fnrmprl hv an 



the spinal cord, which has become thickened and consti- mciaum 1S m en lormea, Dy an 



tutes the peduncles of the cerebrum ; e. tubercula quadri- plpva+irvn rf nprvrmc rnaffpr 

 gemina ; /, optic thalami, covered by the hemispheres. 



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, extends 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 the aque- 

 duct of Sylvius, which is left in the development of the middle vesicle. At 



