n8 VERTEBRATES WITHOUT JAWS iv. 17- 



purpose of respiration. We cannot be certain about changes which 

 occurred so long ago, but it seems likely that the respiratory move- 

 ments of a fish were first introduced to provide food rather than 

 oxygen. 



The endostyle therefore shows the survival of the primitive feeding- 

 methods of chordates, but it also undergoes at metamorphosis an 

 astonishing change into a thyroid gland. The mucus-secreting columns 

 shrink and the whole organ becomes reduced to a row of closed sacs, 

 lying below the pharynx (Fig. 79 b). Each of these sacs is lined by an 

 epithelium, contains a structureless 'colloid' substance, and is there- 

 fore closely similar to a thyroid vesicle. Moreover, experiments have 

 shown that extracts of this organ contain iodine and exert an accelerat- 

 ing effect on the metamorphosis of frog tadpoles. Although nothing is 

 known of the part played by the secretion of this gland in the life of 

 the adult lamprey, we may safely conclude that we have here the 

 conversion of an externally secreting feeding-organ into a gland, of 

 internal secretion. The actual mucus-secreting cells are not trans- 

 formed into those of the thyroid follicles, these latter are derived from 

 epithelial cells in the wall of the larval organ. One cannot avoid specu- 

 lating on this extraordinary change of function. It may perhaps be 

 significant that the endocrine gland that regulates basal metabolism 

 (the thyroid) is derived from the part of the feeding-system that in 

 the earliest chordates was responsible for providing the raw materials 

 of metabolism. Experiments with radioactive iodine show that this 

 element is concentrated in certain cells of the larval endostyle (Fig. 

 79 c). Moreover, after addition of the anti-thyroid substance thiourea 

 to the water there are changes in the endostyle. Thyroxine has been 

 extracted from the gland and it probably has an endocrine function 

 as well as secreting mucus, though no one has ever produced any 

 changes in larval lampreys by administering thyroid hormones. 

 Lampreys thus show, as larvae, a stage in which the accumulation of 

 iodoproteins, previously widespread, becomes concentrated in the 

 pharynx. Perhaps at this site there were already cells specialized for 

 halide transport (cf. the chloride-secreting cells of teleosts, used for 

 osmogulation, p. 203). In adult lampreys and all higher chordates the 

 iodoprotein is secreted into the blood under the control of blood- 

 borne signals (Fig. 80). The change may well be related to develop- 

 ments in the regulation of metabolism, which, in the animals with a 

 fully endocrine thyroid becomes more nearly independent of varia- 

 tions in the external supply of iodine. 



The great change in the endostyle is only part of the complete 



