IV. II 



PITUITARY OF LAMPREYS 



107 



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epithelial layer, corresponding to the pars nervosa of the pituitary of 

 higher forms. The major portion of the pituitary gland is a mass of 

 secreting cells in which two parts can be recognized, the partes 

 anterior and intermedia. After experimental removal of the inter- 

 mediate portion of the pituitary lam- 

 preys become permanently pale in 

 colour (Fig. 69), showing that, as in 

 other vertebrates (p. 299), a melano- 

 phore-expanding substance is liberated 

 into the blood by this gland, the secre- 

 tion being presumably under the control 

 of the pineal eyes (p. 105). The lamprey 

 pituitary has been shown to contain 

 oxytocic and 'water balance' hormones 

 as well as one producing melanophore 

 expansion. Moreover injections of 

 mammalian anterior pituitary extracts 

 induce appearance of the secondary 

 sexual characters of lampreys. Evi- 

 dently the functions of the pituitary 

 have remained essentially the same 

 through the whole chordate series. 



The pituitary of lampreys is peculiar 

 because of great development of the 

 naso-hypophysial sac (Fig. 70). Charac- 

 teristically in vertebrates the pituitary 

 body develops by the formation of a 

 pocket of buccal ectoderm, whose walls 

 then become folded, so that the part 

 in front of the lumen becomes the pars 

 anterior, that behind the pars inter- 

 media. In nearly all vertebrates the 

 lumen then loses its connexion with the 

 exterior. In lampreys the hypophysial 

 rudiment is continuous with that of the 

 olfactory epithelium. The latter then 

 moves dorsally and the two remain 



connected throughout larval life by a strand of cells. At metamorphosis 

 this acquires a lumen and forms a tube extending from the nostril 

 below the pituitary and brain. Because of its development this is 

 sometimes called the naso-hypophysial tube but others doubt that 



Fig. 70. Dissection of lamprey from 

 the ventral surface after injection 

 of coloured gelatine to show the 

 outline of the naso-hypophysial sac 

 (.1) and its duct (d), which is shown 

 dotted where it runs upwards be- 

 tween the nasal sacs («). g, gill 

 pouches. Contraction of the bran- 

 chial apparatus squeezes the sac s, 

 so that water is drawn in at each 

 relaxation. 



