while other properties, such as the ability to stimulate the mammary gland arose
as a result of subsequent changes. That the various characteristics of vertebrate
'prolactins' are indeed due to a fundamentally similar molecule is seen when one
considers their immunochemical behaviour. Anti-mammalian (ovine) prolactin
serum combines with fish 'prolactin', while anti-fish 'prolactin' serum reacts with
ovine prolactin (EMMART and WILHELMI, 1968). Fish 'prolactin' has been termed
'paralactin' by BALL (1965), a term that has received the blessing of others (BERN
and NICOLL, 1968).
2. Water Exchanges
a) The Integument
The skin and gills of fishes are continuously bathed by the environmental solutions
and are avenues for movements of water in or out of the animal. The direction of
any net transfer depends on the direction of the osmotic gradient.
The relative surface areas of the skin and gills vary considerably in different
fishes; in the toadfish, Opsanus tau, the gills have an area 10 times larger than that
of the skin, while in the mackerel, Scomber scombrus, it is 60 times greater (see
PARRY, 1966). The toadfish has recently been shown to have a very low permeability to water and sodium (LAHLOU and SAWYER, 1969) and this may partly
reflect its relatively small branchial surface.
The skin of fishes is a complex structure, which is liberally supplied with
mucous glands; it is often covered with scales, which give it the appearance of a
substantial barrier to molecular transfer. Unfortunately, however, there are few
direct measurements of this. KROGH (1939) after considering the evidence that was
available at the time, concluded that the gills, rather than the skin, are the principal
pathway for the movements of water and ions in and out of fish. There is no subsequent contradictory information. MOTAIS et al. (1969) have recently measured
the movements of tritated water across the gills of several freshwater and marine
fishes (Anguilla, Carassius, Platichthys and Serranus), and found that virtually all
of the water that was exchanged could be accounted for by that which crossed the
gills. The permeability of the gills of these fishes to water was found to be low compared with other epithelial membranes like amphibian skin and urinary bladder.
It was also interesting to observe that the permeability to water was less when the
fish were in sea-water than in fresh water, despite the larger osmotic gradient between the fish and the former solution. The reasons for this difference or change
in the permeability of the membranes is not known. It was suggested that it may
be an example of 'rectification of flow' such as I had observed (BENTLEY, 1961;
1965) in the urinary bladders of toads bathed on their external surface with hyperosmotic solutions. It is thus not necessary to postulate any hormonally mediated
adjustments, but they could occur. BOYLAN (1967) has shown that the gills of
chondrichthyeans are permeable to water and solutes but he also found that this
was somewhat restricted as compared to amphibian membranes.
The agnathan fishes, like their more sophisticated relatives, accumulate water
across their integument. The river lamprey, Lampetrafluviatilis, takes up water
at about the same rate as the goldfish, equivalent to about 33% of the body weight
210
as a result of subsequent changes. That the various characteristics of vertebrate
'prolactins' are indeed due to a fundamentally similar molecule is seen when one
considers their immunochemical behaviour. Anti-mammalian (ovine) prolactin
serum combines with fish 'prolactin', while anti-fish 'prolactin' serum reacts with
ovine prolactin (EMMART and WILHELMI, 1968). Fish 'prolactin' has been termed
'paralactin' by BALL (1965), a term that has received the blessing of others (BERN
and NICOLL, 1968).
2. Water Exchanges
a) The Integument
The skin and gills of fishes are continuously bathed by the environmental solutions
and are avenues for movements of water in or out of the animal. The direction of
any net transfer depends on the direction of the osmotic gradient.
The relative surface areas of the skin and gills vary considerably in different
fishes; in the toadfish, Opsanus tau, the gills have an area 10 times larger than that
of the skin, while in the mackerel, Scomber scombrus, it is 60 times greater (see
PARRY, 1966). The toadfish has recently been shown to have a very low permeability to water and sodium (LAHLOU and SAWYER, 1969) and this may partly
reflect its relatively small branchial surface.
The skin of fishes is a complex structure, which is liberally supplied with
mucous glands; it is often covered with scales, which give it the appearance of a
substantial barrier to molecular transfer. Unfortunately, however, there are few
direct measurements of this. KROGH (1939) after considering the evidence that was
available at the time, concluded that the gills, rather than the skin, are the principal
pathway for the movements of water and ions in and out of fish. There is no subsequent contradictory information. MOTAIS et al. (1969) have recently measured
the movements of tritated water across the gills of several freshwater and marine
fishes (Anguilla, Carassius, Platichthys and Serranus), and found that virtually all
of the water that was exchanged could be accounted for by that which crossed the
gills. The permeability of the gills of these fishes to water was found to be low compared with other epithelial membranes like amphibian skin and urinary bladder.
It was also interesting to observe that the permeability to water was less when the
fish were in sea-water than in fresh water, despite the larger osmotic gradient between the fish and the former solution. The reasons for this difference or change
in the permeability of the membranes is not known. It was suggested that it may
be an example of 'rectification of flow' such as I had observed (BENTLEY, 1961;
1965) in the urinary bladders of toads bathed on their external surface with hyperosmotic solutions. It is thus not necessary to postulate any hormonally mediated
adjustments, but they could occur. BOYLAN (1967) has shown that the gills of
chondrichthyeans are permeable to water and solutes but he also found that this
was somewhat restricted as compared to amphibian membranes.
The agnathan fishes, like their more sophisticated relatives, accumulate water
across their integument. The river lamprey, Lampetrafluviatilis, takes up water
at about the same rate as the goldfish, equivalent to about 33% of the body weight
210