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Electron microscope examinations of the developing triadic junction in fibers from leg muscles of fetal and postnatal rats reveal a range of complexity from no structural connections across the space between apposed membranes of T and SR to all of the junctional structures visible in adult rat muscle fibers. In triads showing no connections, faintly stained projections from the apposed membranes of T and SR are seen. The simplest form of junctional structure is a series of faintly stained strands that make connections across the space between apposing membranes. Junctional structures stained more intensely show periodic bridges traversing the space plus an intermediate dense line (intermediate density) running parallel to the apposed membranes of T and SR and bisecting the space between apposed membranes. The intermediate density is continuous across the periodic bridges and interbridge spaces and it appears to be connected to the apposed membranes of T and SR. It is suggested that projections from T and SR are precursors of the connections and that the projections interact to form the intermediate density. It is also suggested that bridge and interbridge junctional structures might form compartments isolated from the sarcoplasm and thus enhance the flow of electrical current from T to apposed SR by reducing low-resistance shunting of current flow into the sarcoplasm,

The plateau of tetanic tension in a lengthening contraction of muscle at any given length is always greater than that in an isometric contraction at the same given length. To account for this excess of tension, it is suggested that the contractile machinery is "locked" in a strained condition that might make the muscle capable of bearing a greater tension in a lengthening contraction than it can develop in an isometric contraction. It is proposed that this strained condition also occurs in a lengthening contraction of the twitch response. If this proposal is valid it seems certain that the plateu of tension in the lengthening contraction of the twitch response cannot be equated with the plateau of tension in the isometric contraction of tetanus. The design of the original concept, that the plateau of active state tension in the lengthening contraction of the twitch is equal to the plateau of tension (Po) in the isometric contraction of tetanus, is based upon the assumption that the contractile component is neither lengthening nor shortening during both of these plateaus. This assumption is made without considering the possibility that the plateau in the lengthening contraction might be due to concurrent lengthening of series elastic elements and shortening of the contractile component. To account for the plateau of tension observed after quick lengthening in the early phase of twitch contraction indirect evidence is presented favoring the view that the quick lengthening during stretch is followed by slow lengthening and stress relaxation (decrease of tension) in series elastic elements and simultaneous shortening and increase of tension in the contractile component. When the original concept of active state tension in the twitch response is interpreted in the light of lengthening contraction, it is concluded that the labeled or implied Po for the plateau of the active state tension is unwarranted and confusing. It seems that the primary source of confusion is the assumption that the tension a muscle is capable of bearing in a lengthening contraction is equatable with the tension it can develop in an isometric contraction.