SYNOPSIS. Unabsorbed, complement‐inactivated antisera produced in rabbits were used in an immobilization system to study the serology of 31 strains of Tetrahymena pyriformis grown in axenic cultures at 26°C. Fourteen serological “groups” were found: 1. “H”: H, E, S, GC, L3, W, T, GHH, G1‐R and sublines W‐P and T‐P; 2. “PR”: PR; 3. “GL”: GL; 4. “LR”: LR; 5. “L1”: L1 and L2; 6. “GP”: GP and Ch‐S; 7. “WH”: WH6, WH14 and WH52; 8. “N”: N and HS; 9. “Y”: Y and TC; 10. “AA”: AA1, AA2 and AA4; 11. “F”: F and BF; 12. “Gf‐J”: Gf‐J; 13. “EZ”: EZ; 14. “Lava”: Lava. Not all of these “groups” were completely distinct; weak or variable cross‐reactions occurring for particular cultures and antisera during the course of several years of observation suggested a relationship in terms of serotype potentialities, for strains in groups 1 and 2, and in groups 2, 3, 4, 5, and 6. Non‐reciprocal cross‐reactions are tentatively explained in terms of population fluctuations of serotype within particular cultures; several other suggested explanations remain possible. On the whole, however, the strains remained remarkably stable in dominant serotype. Observations on Tetrahymena in paralyzing antisera, with respect to the extrusion of a gelatinous exudate, in general confirmed previous observations by others. “Chain” formation was noted upon recovery, under conditions suggesting that the chains do not always derive entirely from incomplete cytokinesis of dividing organisms but that at least sometimes an aggregation of separate individuals is also involved. Synchronized division was observed for recovering organisms. A gelatinous material was obtained by centrifugation of a large number of individuals; this material provided a degree of protection to the organisms against antiserum effects while other colloidal materials did not, but the protective effects proved nonspecific in the sense that they were not confined to particular strain‐antiserum combinations.

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Rabbits, cats and monkeys show species difference in the consistency and frequency with which spreading depression (SD) can be elicited during the first hours after exposure of the cortex. In the rabbit SD can be produced consistently at 6–10-minute intervals. In most of the preparations of the cat it is also possible to produce a series of SDs when using a longer interval (15–20 min.) between stimulations. Only rarely is it possible to obtain in the monkey a series of SDs even when long intervals are interjected between stimuli. However, the majority of preparations responded at least once with a SD. It has been possible to lead off SD in rabbits and cats with extradural electrodes implanted in the skull 5–7 days earlier. These electrodes were in most instances flush with the dural surface of the skull, resting on the undamaged dura. In the rabbit photo-Metrazol stimulation can produce SD which also has been recorded with the implanted electrodes. Since there is no reason to believe that the cortex in these latter experiments was damaged or in any other way abnormal, the conclusion was drawn that normal cortex can exhibit spreading depression.

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The pars distalis of the pituitary gland of the C57BL mouse was studied by means of electron microscopy during postnatal stages of males ranging in age from newborn through 24 days, with particular emphasis on the somatotrophs and mammotrophs. During this period, growth curves were plotted in order to correlate postnatal growth patterns with the state of differentiation of the somatotrophs in the pars distalis. In the newborn, the somatotrophs show well developed organelles, including rough endoplasmic reticulum and Golgi complexes. These cells are not as densely packed with granules as the adult somatotrophs; however, from days 5 through 24, they show a progressive accumulation of granules. Although mammotrophs are scarce in the newborn, they are readily distinguishable in the pars distalis at 5 days. Male mice in small litters show a progressive increase in body weight between birth and 11-12 days, at which time the rate slackens until 18-19 days when the rate again increases. Growth curves for mice from large litters are similar to those from smaller litters, except that the transitory decrease in rate is more prominent.