THE CENTRAL NERVOUS SYSTEM. 



The anterior lobe, larger and darker than the preceding, for 

 some time remains connected by its tubular ectodermic stalk 

 with the primitive oral cavity ; later the tube becomes atrophic and 

 finally disappears, the end of the oral diverticulum then lying iso- 

 lated and separated from the buccal cavity. The single primary 

 tube undergoes repeated division, producing compartments which 

 appear in the adult organ as slightly convoluted tubular acini. The 

 tubules are held together by vascular connective tissue, and contain 

 polyhedral epithelial cells, with spherical or oval nuclei, irregularly 

 disposed and often almost filling the alveoli ; the lumina of the tubules 

 are sometimes occupied by colloid masses resembling those of the 

 thyroid gland. 



THE PINEAL BODY. 



The pineal body, epiphysis, or conarium, since the compara- 

 tively recent investigations of Spencer and of de Graaf, although 

 known and described previously for centuries, is now regarded as a 

 rudimentary sense-organ. These investigators independently 

 demonstrated that the structure 



seen in man and the higher FIG. 352. 



animals is the rudiment of what 

 was a functionating sense-organ 

 in the extinct reptiles, and even 

 in certain living members of the 

 same class strongly resembles 

 an imperfect invertebrate eye in 

 its early embryonal condition. 

 In the light of our present 

 knowledge, therefore, this pe- 

 culiar body must be looked 

 upon as representing an im- 

 perfect organ of special sense, 

 whether as an additional visual 

 structure the "pineal eye" 

 as an organ for the percep- 



B 



Sagittal section through part of head of lizard 

 embryo, showing so-called pineal eye: P, special- 

 ized isolated extremity of pineal diverticulum from 

 brain-vesicle (B) ; b, c, so-called retinal and len- 

 ticular areas of its walls ; a, ectoderm ; d, remains 

 of diverticulum undergoing division into tubules 

 (d') ; f, blood-vessels ; e, mesodermic tissue. 





tion of warmth still remains to 

 be determined. 



In man and other mammals 

 the pineal body, instead of oc- 

 cupying its morphologically 

 normal position on the superior 

 surface of the brain, is covered 

 over by the greatly developed 

 cerebral hemispheres, so that its final position is well towards the 

 base of the encephalon. The organ at no time in the higher ani- 



