THE INTESTINAL CANAL. 683 



The cavities of these lobes become the lateral ventricles, and are 

 connected by means of the foramen of Monro, which at the earlier 

 periods is very wide, but subsequently becomes narrowed to a 

 mere slit. The cerebral hemispheres are separated by the in- 

 growth of a septum, which is ultimately formed into ihefalx cerebri. 

 The hemispheres are then greatly enlarged in the backward di- 

 rection, so that they quite overlap the thalamencephalon and the 

 parts developed from the middle cerebral vesicle. The corpus 

 callosum is subsequently formed by the fusion of the juxtaposed 

 parts of the hemispheres. 



From the anterior part of the cerebral hemispheres arise two 

 prolongations, which develop into the olfactory bulbs, these grow 

 forwards, and soon lose their cavities which at first communicated 

 with those of the ventricles. 



Middle cerebral vesicle. By the cranial flexure the brain is bent 

 at the junction of the first and second cerebral vesicles, the first 

 cerebral vesicle is" thus turned downwards, leaving the second 

 vesicle as the most anterior part of the brain. 



The upper walls of the second cerebral vesicle are developed 

 into the corpora quadrigemina. 



The cavity of this vesicle persists as a narrow channel, and 

 forms a communication between the third ventricle in front and 

 the fourth ventricle behind, and receives the name in the adult 

 brain of the Her a tertio ad quartum ventriculum. The crura cere- 

 bri arise from the lower wall of this middle vesicle. 



The third cerebral vesicle is divided into an anterior and a pos- 

 terior part. From the upper part of the anterior division arises 

 the cerebellum, and from its lower part the pons Varolii. 



The posterior division gives rise to the medulla oblongata. 



The cavity of this vesicle is called the fourth ventricle. It is 

 continuous with the central canal of the spinal cord. Its upper 

 wall is thinned and forms the valve Vieussens. It communicates 

 with the subarachnoid space through the foramen of Majendie. 



THE ALIMENTARY CANAL AND ITS APPENDAGES. 



When the blastoderm is bent at its anterior extremity to form 

 the cephalic fold, it closes in and forms the anterior boundary of 



