Female budgerigars provided with nest boxes and exposed to male vocalizations were kept on light regimes involving 6 h of light and then a further 2 h of light starting 8, 12, 16, 17 or 18 h after the initial dawn. A higher proportion of females laid when the second light period started 6 h after the first one, than when it started earlier or later. That the effect was not due merely to the males vocalizing more on this light regime than on others was shown by substituting for the males taped vocalizations played during the light periods. It remains possible that females are more responsive to those vocalizations instead of/as well as to light at some points of the circadian cycle than others.
We have used antibodies to three of the isomorphic forms of collagen, types I, III and V, in an immunofluorescence microscopy study of myogenesis in the embryonic chick wing, concentrating on the period between stages 27 and 30 (5 to 7·5 days incubation) which is when the dorsal and ventral muscle masses separate into discrete muscles.
We have demonstrated the presence of all three collagen types at the ectoderm-mesenchyme junction from stage 27 onwards. Type I collagen and then type III collagen are found in progressively deeper layers of the dermis at the later stages. Both types I and V collagen are initially present in the cartilage elements, but type 1 collagen becomes restricted to the periphery of these structures at later stages. The developing muscle areas show a lack of staining at all stages and it is only at the latest stages that types I and III collagen first appear in the surrounding epimysium.
We discuss possible mechanisms for the division of the muscle masses in the light of this information on the distribution of collagen types.
It is well known that the different muscles in the vertebrate limb develop by a series of splittings and subdivisions of dorsal and ventral muscle masses. The mechanism for this process is not clear, and the suggestion from previous studies that tension exerted by the growing limb stimulates these splittings is now thought unlikely. It has also been proposed that nerves play an important part in the separations by physically pushing the muscle mass apart. There is also the possibility that nerves could stimulate differential contraction of parts of the muscle mass, leading to a shearing effect and resulting in separation.
In this study, peripheral nerves are removed by the administration of the nicotinamide analogue 3-acetylpyridine before the start of muscle mass division. The resulting pattern of muscle is normal although nerves are completely absent. This clearly rules out any major role of nerves in the for mation of the muscle pattern, although many authors have shown that innervation is important for the maintenance and later development of the separated muscles.
While the mechanism of the process of division of the muscle masses remains unknown, it issuggested that there are changes in cellular behaviour at areas corresponding to the future spaces between the muscles. These changes may be specified by some aspect of positional information within the limb. They might involve muscle cells stopping or reversing their differentiation as muscle in the areas forming the future spaces, or perhaps change the adhesion Properties of the muscle cells to cause them to ‘sort out’ and separate. Alternatively, a localized invasion of non-muscle mesenchymal cells at the future spaces could lead to separation of discrete muscles. These possibilities are at present under investigation.
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