50 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



the lamina terminalis. On either side of this the fore brain 

 grows outwards, and especially forwards, thus producing two 

 lobes, right and left. These are the cerebral hemispheres, while 

 the rest of the primitive fore brain is the thalamencephalon. 

 The ventricle of the primitive fore brain participates in this out- 

 growth, giving rise to a cavity in each of the lobes ; so that now 

 we have three ventricles in the fore-brain region, the first and 

 second forming a pair, while the third unpaired ventricle remains 

 in the thalamencephalon. The paired ventricles remain in con- 

 nection with the third by small openings, the foramina of 

 Monro. 



"FiG. 52. Sagittal section through vertebrate brain, in front passing through 

 a cerebral lobe, c, cerebrum; cb, cerebellum; h, infundibular (hypophysial) out- 

 growth; m, medulla oblongata; o, olfactory nerves; ol, optic lobes; /, pineal 

 structures; s, spinal cord; 1-4, ventricles. 



While this differentiation is taking place in the fore brain 

 the mid brain (known under various names, mesencephalon, 

 optic lobes, corpora bi- or quadrigemina) remains almost station- 

 ary, the chief change being a thickening of its walls so that 

 (except in teleosts where a part remains as the epicoele) the 

 primitive ventricle of its earlier condition becomes a narrow 

 tube, the iter or aqueduct, 1 connecting the third ventricle with 

 the ventricle in the hind brain. 



In the hind brain the differentiation is largely confined to 

 the dorsal surface. It consists in the outgrowth from the an- 

 terior dorsal wall of a lobe of tissue which extends backward 

 over the rest, and forms the cerebellum or metencephalon, the 

 rest of the hind brain forming the medulla oblongata or myelen- 

 cephalon, which passes into the spinal cord behind. 



The different regions of these five divisions of the brain 

 become variously developed, the walls being thickened in parts 



1 Iter e tertio ad quartnm ventriculttm ; aqueductus Sylvii. 



