. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. THE BRAIN 155 The pineal apparattcs consists of the epiphysis or pineal organ proper, which persists in a more or less rudimentary condition in all Vertebrates, a,nd 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 oi'gan becomes atrophied in the, majority of Vertebrates. Each of these structures represents a vestigial sensory organ, and in certain oases may retain to a greater or less extent the charact
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. THE BRAIN 155 The pineal apparattcs consists of the epiphysis or pineal organ proper, which persists in a more or less rudimentary condition in all Vertebrates, a,nd 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 oi'gan becomes atrophied in the, majority of Vertebrates. Each of these structures represents a vestigial sensory organ, and in certain oases may retain to a greater or less extent the character of a median eyeâ possibly in some degree comparable to that of ' Certain facts seem to indicate that both organs arose primitively in a paired manner. Accessory vesicles occur occasionally in young Slow- worms {), in which as many aS two or even three rudimentary vesicles may be present behind the pineal Fig. 126.âMedian Longitudinal Section thkough the Hfad of a Newly- HATOHED Larva of Petromyzon planeri. (Mainly after Kupfer.) , fore-brain; ;, mid-brain ; , hind-brain ; ep, epiphysis ; li'p, hypophysis ; at, stomodseum ; al, endodermic alimentary cavity ; (A, 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 chordal portion (comp. p. 67). The mid-brain and medulla oblongata undergo fewer modifi- c
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