of the thyroid and gonads (C HADWICK, 1940; 1941). Furt hermore, pituitary transplants from a wide range of other vertebrates could also induce this 'water dr ive'.
Prolactin is also thus effective in hypophysectomized newts, while other mammalian pituitary hormones including growth hormone, th yrotrophin, luteinizin g
hormone, corticotrophin and neurohypophysial extracts are without such an action
(GRANT and GRANT, 1958). Prolactin also can induce Tritu rus cristatus and T. alpestris to seek water in order to breed (TUCHMANN-D uPLESS IS , 1948). Thus a pr olactin -like hormone can initi ate the change from a terrestrial to an aquatic life in
some newts. Whether this hormone also confers any changes which directly influence osmoregulation are unknown. Morphological changes take place in the skin
of such newts and these conceivably could influence its permeability to water and
salts. The responses of the three life stages of Diemictylus to vasotocin have been
measured; all responded by accumulating water but, while this was less in the larval
form, it was similar in magnitude in the terrestrial efts and aquatic 'adults' (OVERACK and DERoos, 1967). Current interest in the po ssible osmoregulatory effects
of prolactin are centered on its effects in certain euryhaline fishes . In jections of
this hormone as we shall see, promote survival of such fish .after the y have been
hypophysectomized and placed in fresh water.
6. Osmoregulation in Larval Amphibians
The larvae of Amphibia live an exclusively aquatic existe nce and even bre athe with
the aid of gills. They thu s sho w more ph ysiological affinities with their ancestor s,
the fishes, than do adult amphibians.
Anuran larvae, or tadpoles, usuall y live in fresh wa ter, though those of R ana
can crivora live in the sea (G ORDON et al., 1961). The larvae of the bullfrog, Rana
catesbe iana , may attain a weight of nearl y 20 g and hence have been especiall y favoured by the ph ysiologists fo r the study of tadpole osmoregulation. The availabl e
information is, nevertheless, rather incomplete. Bullfrog tadpoles read ily exchange
sodium with the solutions that bathe them, but the influx and outflux of this ion
are about equal and there is no evidence of an active accumulation by the animal
other than by feeding (ALVARADO and JOHNSON, 1966). TAYLOR and BARKER
(1965) measured the electrical p.d. across the isolated skin of these tadpoles and
found it, in contrast to adult bullfrogs, to be zero in young larvae and did not begin
to develop until metamorphosis was about to occur. This also suggests (but not con -
clusivel y) that active ion transport does not take place acros s the skin of young
tadpoles. This deficiency has also been correlated with the low levels of Na-K activated ATPase in the skin of the larvae (KAWADA, TAYLOR, and BARKER, 1969).
The rate of water uptake in v ivo is unknown, but the skin of tadpoles (in v itro)
is osmotically permeable, water moving across it at the rate of about 3,ullcm
2h,
which is only about 25% as fast as in bullfrogs (BENTLEY and GREENWALD, 1970).
Presumably this water and any that is taken in through the gut, is normally excreted
by the kidneys.
More recently ALVARADO and MOODY (1970) have shown th at bullfrog tadpoles which have been salt-depleted (in distilled water) can activel y accumulate
sod ium and chloride. This results mainly from a facilitated influx of sodium but
187
Prolactin is also thus effective in hypophysectomized newts, while other mammalian pituitary hormones including growth hormone, th yrotrophin, luteinizin g
hormone, corticotrophin and neurohypophysial extracts are without such an action
(GRANT and GRANT, 1958). Prolactin also can induce Tritu rus cristatus and T. alpestris to seek water in order to breed (TUCHMANN-D uPLESS IS , 1948). Thus a pr olactin -like hormone can initi ate the change from a terrestrial to an aquatic life in
some newts. Whether this hormone also confers any changes which directly influence osmoregulation are unknown. Morphological changes take place in the skin
of such newts and these conceivably could influence its permeability to water and
salts. The responses of the three life stages of Diemictylus to vasotocin have been
measured; all responded by accumulating water but, while this was less in the larval
form, it was similar in magnitude in the terrestrial efts and aquatic 'adults' (OVERACK and DERoos, 1967). Current interest in the po ssible osmoregulatory effects
of prolactin are centered on its effects in certain euryhaline fishes . In jections of
this hormone as we shall see, promote survival of such fish .after the y have been
hypophysectomized and placed in fresh water.
6. Osmoregulation in Larval Amphibians
The larvae of Amphibia live an exclusively aquatic existe nce and even bre athe with
the aid of gills. They thu s sho w more ph ysiological affinities with their ancestor s,
the fishes, than do adult amphibians.
Anuran larvae, or tadpoles, usuall y live in fresh wa ter, though those of R ana
can crivora live in the sea (G ORDON et al., 1961). The larvae of the bullfrog, Rana
catesbe iana , may attain a weight of nearl y 20 g and hence have been especiall y favoured by the ph ysiologists fo r the study of tadpole osmoregulation. The availabl e
information is, nevertheless, rather incomplete. Bullfrog tadpoles read ily exchange
sodium with the solutions that bathe them, but the influx and outflux of this ion
are about equal and there is no evidence of an active accumulation by the animal
other than by feeding (ALVARADO and JOHNSON, 1966). TAYLOR and BARKER
(1965) measured the electrical p.d. across the isolated skin of these tadpoles and
found it, in contrast to adult bullfrogs, to be zero in young larvae and did not begin
to develop until metamorphosis was about to occur. This also suggests (but not con -
clusivel y) that active ion transport does not take place acros s the skin of young
tadpoles. This deficiency has also been correlated with the low levels of Na-K activated ATPase in the skin of the larvae (KAWADA, TAYLOR, and BARKER, 1969).
The rate of water uptake in v ivo is unknown, but the skin of tadpoles (in v itro)
is osmotically permeable, water moving across it at the rate of about 3,ullcm
2h,
which is only about 25% as fast as in bullfrogs (BENTLEY and GREENWALD, 1970).
Presumably this water and any that is taken in through the gut, is normally excreted
by the kidneys.
More recently ALVARADO and MOODY (1970) have shown th at bullfrog tadpoles which have been salt-depleted (in distilled water) can activel y accumulate
sod ium and chloride. This results mainly from a facilitated influx of sodium but
187