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Developmental problems relating to the stability of cell phenotype, and to the interactions between cells during morphogenesis can be approached particularly well through the study of regenerating systems. In the case of lens regeneration the anatomy of the eye is such that it is possible to identify the tissue types that can form lens, and to analyze the role of the factors which control the regenerative process. At present, it has been established that the lens can regenerate from the iris (Stone, '59; Reyer, '54, '62); and there is some evidence that the lens can regenerate from the cornea (Reyer, '54).The most convincing evidence for lens regeneration from the cornea has been provided by Ikeda ('36, '39) for late embryonic and early larval stages of Hynobius unnangso. In this series of experiments, the lens anlage with the surrounding ectoderm was removed from embryonic animals and head ectoderm was allowed to heal over the eye cup. A histological study of these animals showed various stages of lens formation from the ectoderm over the eye cup, but a complete series of stages showing the process of lens regeneration was not presented. The time during which lens regeneration could take place was limited to a short period of embryonic life after lens induction. These results were complicated by the claim that lens regeneration from the iris takes place during the same time period (Ikeda, '37). In the second experiment in this series pieces of prospective and differentiating cornea from animals at different developmental stages were implanted into the eye cup of young animals after lens removal. Here lens formation was observed from the inner cell layer of the epithelial component of the cornea. The portion of the corneal epithelium which did not contribute to the lens usually developed into epidermis. This paper describes lens regeneration from the cornea in another species, XenoUniversity of Chicago, Illinois p u s laevis. The study was made on three different larval stages, and young metamorphosed animals. With this material the process of lens regeneration was studied histologically, the growth of the lens was plotted, and the competence of the eye to regenerate a lens at different developmental stages was measured. A cytological analysis was then made of the changes in the behavior of the nucleolus that take place during regeneration.This description of lens regeneration was supplemented by experiments designed to verify the observation that the cornea forms lens. This was done with extirpation and grafting experiments, in some of which tritiated thymidine was used as a marker.Two factors which control the process of lens regeneration were also studied. The role of the optic cup was studied by testing for lens regeneration in its absence. The limitation of regenerative capacity to the larval eye was studied by local application of thyroxin.A preliminary account of this work has been published (Overton and Freeman, '60; Freeman and Overton, '61, '62). MATERIALS AND METHODS AnimalsThe animals used for the e...

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The purpose of this study was to determine whether the amount of alveolar epithelial tissue damaged during exposure of NO2 could be quantified by measuring the proliferative response to Type 2 cells. To accomplishe this, we used tissues from previously published experiments in which rats had been exposed to NO2 and the proliferative response to Type 2 cells had been measured during a 5-day period. The proportion of alveolar epithelium damaged was determined by stereologic examination with electron microscopy of tissue sections from those rats exposed to NO2 for 24 hours. These values were then compared with the total proliferative response to Type 2 cells for the 5 days of exposure. The study demonstrated that increasing tissue damage is assocaited with a greater proliferative response to Type 2 cells. The high degree of correlation (r = 0.93) indicates that the proliferative response of Type 2 cells can be used as an indirect means to quantify acute damage to the alveolar epithelium.

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