The embryo ofa tarsonemid mite was found to be suitable for in vivo observations of muscle development by polarization microscopy. The four dorsal muscles of the metapodosoma each contain three sarcomeres, the anterior two of which can be seen clearly. These sarcomeres can be identified and followed during much of their development. Sarcomeres are about 2. 5 micra long when first detected and increase in length until they are about IO micra long. The change in length is associated with a slow, approximately constant rate of increase in the length of the A region, and an initially slow then much more rapid increase in the length of the I band. Preceding the period when the I band elongates rapidly there is an increase in the diameter of the muscle fibers and an increase in the retardation of the A band. A, I, Z, and H bands are visible during most of these changes. The change in A band length has been interpreted in terms of the growth of the A filaments which have been observed by electron microscopy in muscles of other animals. It is suggested that the exceptionally long sarcomeres in this mite result from the early fixing of the number of sarcomeres in a given muscle fiber.The development of the striated muscle sarcomere is poorly understood despite many investigations of muscle differentiation. With light microscopy a uniform filamentous structure has been seen before striations are observable (1-4), but investigations by electron microscopy (5, 6) indicate that the sarcomere period may be present before A bands are detectable. Details of A, I, and H band formation have not been described.Polarization microscopy is especially suitable for the study of some aspects of muscle structure in vivo. Muscles thin enough for good resolution by polarization microscopy, using a rectified (7) lens system, were observed in a tarsonemid mite. During a period of about 36 hours preceding the first contractions, changes in sarcomere length and in A and I band length could be followed in the living animal. METIIODSMites collected and identified by Heinemann 1 as Tarsonemus randsi were kept in culture (8) on Fusarium oxyporum at 25°C. Embryos selected when muscles are first apparent may be flattened, and most will develop and hatch when mounted in mineral oil which has been saturated with water.Observations were made with a X 97, N.A. 1.25, strain-free immersion objective, 2 using a model P-42 American Optical Company polarizing microscope? The substage of this microscope was modified to 1 I am indebted to Mr. Richard Heinemann for a culture of Tarsonemus randsi and for advice on maintaining the culture. 2 American Optical Company. These lenses were specially selected for their strain-free qualities.

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Human retinal pigment epithelium (RPE)-derived cell lines were established from RPE-covered choroid tissue fragments, which had been generated by culture on nontissue culture plastic. Two phenotypes were apparent in a given line: (a) a compact cell which formed domes and ultimately melanosomes before being sloughed; and (b) a squamous cell which was often elongated and which bound antibody to human keratins. This latter cell did not become black or form domes. The average number of cell doublings for the 13 lines tested was between 15 and 40 when cultured in a modified Eagle's minimum essential medium containing 10% fetal bovine serum. Cell lines newly established from material that had been in culture for more than 6 months had normal mitotic chromosomes and still developed areas with strongly pigmented cells when refed. Normal human epithelial cell lines of this kind may be useful in studies of cell aging and defining change associated with the development of neural cells from ectoderm.

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