DEVELOPMENT OF THE NERVOUS SYSTEM. 401 



The cerebrum, as we have just seen, is developed from 

 the anterior division of the first primitive cerebral vesi- 

 cle. The development of this part is more rapid in its lat- 

 eral portions than in the median line, which divides the cere- 

 brum 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 cerebel- 

 lum. Up to 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. 1 The sep- 

 tum lucidum is then formed by an elevation of nervous mat- 

 ter 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 devel- 

 oped 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 de- 

 veloped into the cerebellum, the convolutions of which ap- 

 pear 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 



1 LONGET, Traite de physiokgie, Paris, 1869, tome iiu, p. 891. 



