824 



EMBRYOLOGY 



tudinal fissure. At the same time, by the rapid development of the 

 posterior portion, it extends over the optic thalami, the corpora quad- 

 rigemina and the cerebellum. Until the end of the fourth month, the 

 hemispheres 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 rapidly 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 di- 

 vision of this first primitive 

 vesicle forms the optic thai- 



Fig. 234. Development of the spinal cord and brain of 

 the human subject (Longet, after Tiedemann). 



A, brain and spinal cord of an embryo of seven weeks, 

 lateral view. 



B, the same, from an embryo further advanced in develop- 

 ment ; b, spinal cord ; d, enlargement of the spinal cord, 



with its anterior curvature; ,, cerebellum ; * tubercula rem ain Connected together 

 quadngemina ; /, optic thalamus ; g, cerebral hemi- 

 spheres. 



C, brain and spinal cord of an embryo of eleven weeks; 

 d, spinal cord ; d, enlargement of the spinal cord, with 

 its anterior curvature ; c, cerebellum ; e, tubercula quad- 

 rigemina; g, cerebral hemispheres ; o, optic nerve of the 

 left side. 



ami. These become sepa- 

 rated in front into two 

 lateral halves, but they 



at their posterior portion, 

 which becomes the posterior 

 commissure. The central 

 canal of the cord is pro- 



C', the same parts in a vertical section in the median line, longed Upward between the 

 from before backward ; b, membrane of the spinal cord, . . 

 turned backward; d, second curvature of the upper OptlC thalami and forms the 

 portion of the spinal cord which has become thickened third ventric le, which is COV- 

 and constitutes the peduncles of the cerebrum ; <?, tuber- 

 cula quadrigemina; / optic thalami, covered by the ered by the hemispheres. 



The second, or middle 



cerebral vesicle, becomes filled with medullary substance, extends up- 

 ward 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 bulb, in the substance of which 

 is the fourth ventricle, communicating with the third ventricle by 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 Varolii. 



In Fig. 234 (Cy o\ it is seen that the vesicles for the organs of vision 



