THE BRAIN 



155 



The pineal apparatus consists of the epiphysis or pineal organ 

 proper, which persists in a more or less rudimentary condition in 

 all Vertebrates, and of a more anterior outgrowth which may be 

 called the parietal organ, arising from the epiphysis or indepen- 

 dently from the roof of the thalamencephalou ; the latter organ 

 becomes atrophied in the majority of Vertebrates. Each of these 

 structures represents a vestigial sensory organ, and in certain cases 

 may retain to a greater or less extent the character of a median eye 

 possibly in some degree comparable to that of Tunicates. 1 



Certain facts seem to indicate that both organs arose primitively in a 

 paired manner. Accessory vesicles occur occasionally in young Slow- 

 worms (Ang-uis), in which as many as two or even three rudimentary vesicles 

 may be present behind the pineal organ. 



FIG. 126. MEDIAN LONGITUDINAL SECTION THROUGH THE HEAD OF A NEWLY- 

 HATCHED LARVA OF Petromyzon planeri. (Mainly after Kupfer.) 



J".b, f ore -brain ; m.b, mid-brain ; h.b, hind-brain; ep, epiphysis; hp, hypophysis; 

 st, stomodeeum ; al, endodermic alimentary cavity ; ch, notochord. 



The hypophysis apparently represents a glandular organ, the 

 secretion of which formerly passed into the ventricles, and various 

 hypotheses have been put forward as to its first origin. 



One of the more recent of these theories assumes that it corresponds to the 

 primitive mouth (palceostoma) of the Proto-Vertebrata, which is to a greater 

 or less extent represented by the combined unpaired nasal and pituitary 

 passage of Cyclostomes (see under Olfactory Organ) : the mouth of existing 

 Vertebrates must then be distinguished as a neostoma. 



Both the primary and the secondary fore-brain are situated in 

 the pre-chordal region of the skull, all the other divisions of the 

 brain lying in its chord al portion (comp. p. 67). 



The mid-brain and medulla oblongata undergo fewer modifi- 

 cations than the fore-brain ; only the anterior part of the thin 



- - a Still more anteriorly a third outgrowth or paraphysi*, arising from the 

 secondary fore-brain, has been observed in the embryos of various Vertebrates. 



