The postnatal development of stereocilia was studied in the Syrian golden hamster. The purpose was to describe the morphological changes underlying the differentiation of stereocilia bundles and to define the time course of their growth in different regions of the cochlea. Differentiation of the hair bundle occurred by progressive changes in stereocilia number, dimensions, and spatial relationships. The overall transformation of the bundle is interpreted as a four-stage process involving the initial production of stereocilia (stage I), differentiation into tall and short populations (stage II), formation of distinct ranks (stage III), and resorption of excess stereocilia (stage IV). The orientation and arrangement of stereocilia during stage II began to occur before the tectorial membrane grew over the hair cell field. Growth in the dimensions of stereocilia occurred continuously throughout these four stages with increases in length and width occurring simultaneously. The time frame of the growth process depended both on location along the organ of Corti and on the type of hair cell. Hair bundles in the basal turn began growing and reached maturity a few days earlier than those in the apical turn. Stereocilia of outer hair cells matured earlier than those of inner hair cells. Outer hair cell stereocilia reached their adult lengths by 14 days after birth, those of inner hair cells between 16 and 18 days after birth. A kinocilium was present on almost all hair cells on the day of birth, but most were eliminated by 14 days after birth. Tip links were observed as early as 4 days after birth, and their growth appeared to be synchronous with the growth of stereocilia. The spatial gradient of stereocilia length, which increased toward the apex in the adult, was nearly the reverse of that seen at birth. The gradient for inner hair cells was associated with a gradient in the rate of stereocilia growth. The data further expand the foundation for interpreting mechanisms underlying the morphogenesis of stereocilia bundles in mammals and for understanding structure-function relationships during development.

Levels of 1 I -0xotestosterone (1 7 P-hydroxyandrost-4-ene-3, I 1 -dione) and testosterone in the blood serum of individually marked adult male Atlantic salmon held in captivity, were measured by radioimmunoassay at approximately monthly intervals for periods of up to 18 months. In addition to peak concentrations of both hormones shown by all the maturing fish at the time to full sexual maturation during October and November, a majority of maturing fish also showed a significant elevation of 1 1 -o,xotestosterone during the early months of the year. The possible involvement of this early elevation of 1 1-oxotestosterone in controlling the mitotic multiplication of spermatogonia is discussed. Weight and length increases expressed as specific values G w and GL and weight to length relationships for the maturing males for each sampling period are presented and compared with those of nonmaturing fish.

Peaks in the emigratory activity of sexually immature, juvenile Atlantic salmon, Sulmo sulur, occur in autumn and early winter and in spring at the Girnock Burn in Aberdeenshire. Stream temperature is apparently without effect on the release of potentially emigrant fish from the stream. Migrants tend to leave the stream when stream discharge rate is elevated. Whereas downstream movements in autumn and the earlier part of the spring are made almost invariably when the stream's discharge rate is higher than the seasonal norm, later in spring they often occur when discharge rates are lower than the seasonal norm and when no apparent increase in absolute stream discharge rate has occurred. Emigration is depressed in the lunar quarter centred on the full moon. The nature of the relationship between the autumn and spring emigrations is discussed.

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The histology of the spurdog (Squalus acanthias L.) testis was observed through a complete annual cycle. The maximum sperm accumulation coincided with the breeding time as defined by the biennial female cycle. Degeneration of spermatogonia was observed following breeding. The band of degenerate germinal ampullae was identical to that described as occurring in Scyliorhinus canicula (L.) following pituitary ventral lobectomy. This suggested a resting phase in the production of a specific pituitary gonadotrophin lasting for about four months. All spermatocytes were transformed to spermatozoa within 12 months. 3β-Hydroxysteroid dehydrogenase activity was located in semen, in Sertoli cell cytoplasm and in the lumens of mature germinal ampullae. A possible seasonal change in this activity due to the migration of the degenerate band is suggested. Interstitial cells of Leydig could not be observed.INTRODUCTIONThe Scottish-Norwegian stock of the spurdog, Squalus acanthias L., sampled for this study, was shown by Holden (1965) to make an autumn breeding migration from the region of the Orkney and Shetland isles to the breeding areas in the Minch off north-west Scotland. During January and February those females carrying embryos at the end of their second year of development move inshore and liberate their young in shallow water. At this time the ovaries of these fish contain from three to fifteen eggs ready for ovulation (Holden & Meadows, 1964), and observations suggest that copulation, ovulation and the commencement of a further pregnancy follow almost immediately. Following copulation, the spermatozoa must reach the oviducal gland at the head of the oviduct in order that the eggs may be fertilized before they are enclosed by the candle membranes.