4 68 



STRUCTURE OF VERTEBRATA. 



become much thickened (optic thalami) ; the thin floor 

 gives off a slight ventral evagination, or infundibulum, 

 which bears the enigmatical pituitary body or hypophysis. 



The pituitary body. This is derived in part from the brain and 

 in part from the mouth, and is extremely difficult to understand. It 



is apparently equivalent in part to the sub- 

 neural gland of Tunicates, but this does not 

 carry us much further. Dohrn connected it 

 with two abortive gill-slits, but the evidence 

 seems insufficient. Beard has interpreted it 

 as a residue of the original mouth which 

 Vertebrates are supposed to have possessed 

 before the persistent one with which we are 

 familiar was evolved, and of the innervation 

 of that hypothetical structure ; but again con- 

 firmation seems wanting. Of its physiological 

 nature we know almost nothing, beyond that 

 a pathological state of this organ is asso- 

 ciated in man with certain diseases, e.g. 

 acromegaly. 



The pineal body. The dorsal upgrowth 

 (or epiphysis) from the roof of the thalamen- 

 cephalon is represented, though to a varying 

 extent, in all Vertebrates. It is terminally 

 differentiated into a little body known as the 

 pineal body. This was entirely an enigma 

 until De Graaf discovered its eye-like structure 

 in Align is, and Baldwin Spencer securely 

 confirmed this in the New Zealand " lizard " 

 (Sphenodon), where the pineal body shows 

 distinct traces of a retina. 



In Elasmobranchs the pineal process is 

 very long, and, perforating the skull, termin- 

 ates below the skin in a closed vesicle. In 

 the young frog it also comes to the surface 

 above the skull, but degenerates in adolesc- 

 ence. In Sphenodon the stalk passes through 

 the skull by the " parietal foramen," so that 

 the "eye" itself lies close beneath the skin, 

 the scales of which in this region are special- 

 ised and transparent. In 1 'guana, Angnis, 

 Lacerla, etc., the epiphysis loses connection 

 with the "eye" portion; and it is also to 

 be noticed that in Angnis and Iguana the 



pineal body receives a nerve from a "parietal centre" near the base, 

 but independent of the epiphysis ; this nerve is transitory in Angnis, 

 more or less persistent in Iguana. Above Reptiles the pineal stalk 

 is always relatively short, and its terminal portion forms a glandular 

 structure. In fact, the development of the pineal body is much 



FIG. 227. Origin of 

 pineal body. After 

 Beard. 



Lowest figure a section 

 through the first embry- 

 onic vesicle, while the 

 medullary groove (g.) is 

 still open ; o., optic out- 

 growths. Middle figure 

 shows beginning of 

 pineal outgrowth (/.). 

 Topmost figure shows 

 a later stage. 



