176 GENERAL ZOOLOGY 



The cavities of the cerebral hemispheres constitute the first 

 and second ventricles, each of which is connected by a narrow 

 passage, the foramen of Monro, with the third ventricle, which is 

 the cavity of the diencephalon. A narrow channel, called the 

 iter or aqueduct, extends through the mesencephalon, connecting 

 the third ventricle with the fourth ventricle lying in the medulla. 

 The roof of the fourth ventricle is very thin and is easily torn in 

 dissecting the brain. The cavity beneath it is broad and shallow. 

 In lower vertebrates the iter is expanded dorsally to form the 

 mesocoel in the region of the mesencephalon. 



Pituitary Gland. — The pituitary gland of the adult vertebrate 

 is composed of two general regions, known as the anterior and 

 posterior lobes. The anterior lobe, or hypophysis, develops as an 

 evagination of the dorsal part of the embryonic oral cavity. The 

 posterior lobe is formed from an evagination of the floor of the 

 diencephalon known as the infundibulum. In all vertebrates 

 except some of the cyclostomes the hypophysis loses its connec- 

 tion with the oral cavity and unites with the infundibulum, which 

 remains connected with the brain throughout life. Thus the 

 anterior lobe of the pituitary gland is derived from the oral 

 epithelium and the posterior lobe from the neural tube. The 

 intermediate lobe of the pituitary gland is proliferated from cells 

 of the anterior lobe and forms a more or less distinct part between 

 the anterior and posterior lobes. 



Parietal Organ and Pinealis. — Two structures, varying con- 

 siderably in different vertebrates, develop from the roof of the 

 diencephalon. The more anterior of these, the parietal organ, is 

 best represented in the lower vertebrates and lizards, in some of 

 which it is provided with a lens and retina, though it has not been 

 established that visual function is present. In other vertebrates, 

 such as amphibia and mammals, the parietal organ is more 

 rudimentary or lacking. The pinealis is a slender outgrowth in 

 the mid-line, posterior to the parietal organ. In some of the 

 lower vertebrates its structure is like that of an eye, but in the 

 higher forms it takes on a glandular character. In the develop- 

 ment of the pinealis of the frog, the distal end is cut off to form the 

 brow spot. Apparently the parietal organ and the pinealis 

 represent degenerated median unpaired eyes. 



Meninges.— The brain and spinal cord of the frog are sur- 

 rounded by two connective tissue membranes called the meninges. 



