No abstract

The ampullate silk gland of the spider, Araneus sericatus, produces the silk fiber for the scaffolding of the web . The fine structure of the various parts of the gland is described . The distal portion of the duct consist of a tube of epithelial cells which appear to secrete a substance which forms the tunica intima of the duct wall. At the proximal end of the duct there is a region of secretory cells . The epithelium of the sac portion contains five morphologically distinct types of granules . The bulk of the synthesis of silk occurs in the tail of the gland, and in this region only a single type of secretory droplet is seen in the epithelium . Protein synthesis can be stimulated by the injection of 1 mg/kg acetylcholine into the body fluids . 10 min after injection, much of the protein stored in the cytoplasm of the epithelial cells has been secreted into the lumen . 20 min after stimulation, the ergastoplasmic sacs form large whorls in the cytoplasm . Protein, similar in electron-opacity to protein found in the lumen, begins to form in that portion of the cytoplasm which is enclosed by the whorls . The limiting membrane of these droplets is formed by ergastoplasmic membranes which lose their ribosomes .

Purified human globin injected into rats forms aggregates which are identifiable by their characteristic appearance in thin sections in the electron microscope and by their positive autoradiographs when the globin is tritiated before injection. Globin is taken up by endothelial cells of glomerular capillaries and is transported across the cell within the limits of a surrounding membrane. Globin is rarely seen to pass through fenestrations. Globin is also taken into the stalk region where it is seen usually within the sponge fibers and only occasionally within stalk cells. Globin is seen in all stages of passage through the basement membranes and sponge fibers, which are not deformed by its passage. On the basis of the findings presented here and by others, it is postulated that the basement membrane and sponge fibers consist of a thixotrophic gel. After traversing the basement membrane, the globin passes between foot processes of the epithelial cells. The slit membranes are deformed by this passage and thus appear to be distinctive structures. The globin is next found free in Bowman's space; the earliest aggregates are seen there within 1 minute after injection. Globin taken up in the stalk region is slowly discharged and very little is found there 6 hours postinjection.

Spikes were recorded from single axons in the siphonal nerve of the hardshell clam Mercenaria mercenaria which respond to dimming of light. No axons were found to respond to the onset, or increase, of illumination. In a dark-adapted state there is little or no ongoing spike activity. The responsive area of a single axon is a circle of approximately 85 um diameter on the inner siphon wall. The number of spikes elicited at the off of constant-duration flashes grows as approximately the 0.4 power of flash intensity. For constant intensity and constant light-time fraction, the off-response increases with increasing duration at least up to 500 s duration. For long durations, the response grows as the logarithm of stimulus duration. Subthreshold light can suppress the off-response from preceding illumination. In a light-adapted state, the offresponse is greater and its latency shorter than in the dark-adapted state. The fine structure of groups of cell processes thought to comprise the photoreceptor in Mercenaria is described. On the basis of morphological and physiological findings it is suggested that phototransduction occurs in the fine distal processes of the axons from which we have recorded. Axonal processes were found to contain well organized pentalaminar whorls which may be the site of photopigment concentration. The action spectrum obtained from the integrated responses of nerve bundles appears to be that of a single Dartnall pigment having maximal absorption at about 510 nm.