STRUCTURE OF THE VERTEBRATES 291 



a primitive retina and lens. The eye extends to the surface 

 through a median parietal foramen. Its highest development is 

 seen in the most primitive living reptile (Sphenodon of New 

 Zealand), but there is no proof that it is capable of visual func- 

 tion. The mammals show no evidence of a dorsal eye, the organ 

 being glandular in structure and deeply hidden between the 

 cerebral hemispheres. This pineal gland is discussed further in 

 Chapter XIX. 



The ventral outpocketing is the infiindibuliim. As it grows 

 ventrally (in the vertebrates above the cyclostomes) it is met 

 by an invagination of ectoderm from the stomodeal region. 

 The stalk connecting the latter with the mouth is eventually 

 cut off by the developing bones of the skull, and the combined 

 organ becomes the pituitary gland which rests in the sella 

 turcica of the skull. In the cyclostomes there is an infundibular 

 evagination from the brain, but the anterior pouch (known in 

 the embryology of the higher vertebrates as Rathke's pouch) 

 remains separate, and is the naso-pituitary sac. In the lamprey 

 the sac ends blindly, its tip lying immediately below the infun- 

 dibulum. 



Midbrain. The midbrain persists in a less modified condition 

 than any primary part of the brain. In all classes except the 

 mammals this region becomes the thick-walled, bi-lobed optic 

 lobes. These lobes are the center of visual sensations. The optic 

 vesicle arises from the forebrain, but the nerves which develop 

 from the retina grow backward into the midbrain and carry light 

 stimuli to the nuclei of the optic lobes. 



The lobes are secondarily divided in the mammals by a 

 transverse fissure, and the four bodies are called the corpora 

 quadrigemina. The nuclei relay optic and auditory sensations 

 from the organs of special sense to the cortex, the major centers 

 of sight and hearing in the mammals being located in the 

 cerebrum. 



The ventricle of the midbrain is fairl}^ large in the dogfish and 

 widely connected with the third ventricle of the forebrain; but 

 in the higher groups the growth of nuclei and the heavy bands 

 of fibers which pass ventrally as the cerebral nuclei increase, 

 gradually crowd out the ventricular space. In the mammals this 



