52 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



(educabilia) fissures appear in its surface, separating convolu- 

 tions or gyri, and the higher the mammal the more numerous 

 the convolutions. It will readily be seen that this produces an 

 increase in surface, and consequently of cortical (ganglionic) 

 substance ; and it is noteworthy that this increase is correlated 

 with increase of intelligence. 



From its anterior ventral region each hemisphere gives off 

 an olfactory lobe (rhinencephalon) into which a part of the ven- 

 tricle may extend. Connected with each olfactory lobe is an 

 olfactory ganglion which may be placed either in the cerebrum 

 itself, or may be carried out towards the end of the olfactory 

 lobe. From these lobes arise the olfactory nerves (see below). 

 In the diencephalon the lateral walls become thickened into 

 large tracts, the thalami, while the dorsal wall as a whole retains 

 its epithelial character, becoming variously folded to form the 



anterior choroid plexus, which car- 

 ries blood-vessels into the three 

 anterior ventricles. From this 

 dorsal surface are also developed 

 three structures, pinealis, epi- 

 physis, and paraphysis, to be 

 mentioned again in connection 

 with the sense organs. From 

 FIG. 55. Sagittal section through the pinealis a pair of nerve tracts, 

 head of larval Petromyzon, after the habenulae, run along the inner 



von Kupffer. c, notochord ; h. in- i c i 1 i -r-i 



vacation for hypophysis; i, infun- SldeS <* the thalam1 ' The fl r 



dibulum ; m, mouth cavity ;, nasal of the thalamencephalon gives 



involution ;./, pineal outgrowth. r j se to a hollow Outgrowth, the 



infun dibulum, which extends 



backwards and downwards, developing from its extremity tis- 

 sue, which unites with other cells, derived directly from the 

 ectoderm, to form the hypophysis or pituitary body. This ecto- 

 dermal portion arises from the ectoderm between the nose and 

 the mouth, or from the mouth itself, and grows upwards and 

 inwards to join the infundibular portion. For a time it retains 

 its connection with the parent layer by means of a cord of cells, 

 the hypophysial duct, which later disappears. The significance 

 of these ventral structures of the tvvixt brain is very obscure. 



