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Histological changes in the perch ovary during development are described and related to changes in gonad weight and to a macroscopic scale of maturity stages after Kesteven (1960). A single organ formed by the fusion of two primordial ovaries, a central internal oviduct, and an extremely thick chorion surrounding oocytes are characteristic features of the perch ovary. Resting oocytes can be differentiated from developing oocytes which are larger and possess a granulosa layer which permits formation of the chorion and yolk acquisition. In the maturing ovary, oocyte diameter increases rapidly and the chorion, which possesses four layers, the tunica propria, two middle zones forming a fine network, and the zona radiata, also expands. The spent ovary is disorganized with follicles and tunica wall contracted, and a number of residual oocytes may be apparent. There was no evidence of pre-ovulatory degeneration although autolysis of a small number of residual oocytes was observed. The gonadosomatic index was highest for males in September and at a maximum in females immediately prior to spawning. After spawning the index fell rapidly with lowest values recorded in June and July. Variations in the gonadosomatic index are related to developmental stages in females.

(Text-figs. 1-8) 591 The herring (Clupea harengus L.) deposits its eggs in the coastal waters around the North Atlantic Ocean, North Sea, and Baltic Sea. The salinity on the spawning grounds may vary from about 35%0 to 5%0' For instance, Brandhorst (1959) reports that successful spawning took place in the Kiel Canal in salinities down to 5%0' and Ford (1929) records that ripe herring have been found in the Tamar estuary. In a series of experiments Ford found that the eggs of herring could be successfully fertilized and incubated even in a salinity of 4.8%0' McMynn & Hoar (1953) investigated the effect of salinity on the development of the Pacific herring C. pallasii and found it had a wide tolerance, the lower level being somewhere between a and 6%0'The experiments now described were carried out as part of an investigation into osmoregulation in the herring, and particularly to find the range of conditions which herring eggs and larvae might tolerate both in nature and in rearing tanks. An extension of this work into the temperature tolerance of herring larvae is described by Blaxter (1960). MATERIALS AND METHODSRipe gonads were obtained from herring caught in the Firth of Clyde in February and off the Scottish East Coast in September and the eggs fertilized in the laboratory. The techniques of storage and fertilization are described by Blaxter (1955). After fertilization, eggs were incubated in sea water of salinity 31-32%0 at temperatures of II'2-II'7°C in 50 1. glass tanks. Smaller numbers of larvae were incubated in water of other salinities in 500 m1. jars; the water in these jars was changed every second day. Low salinity water was made up by adding distilled water to sea water and high salinity water by adding sodium cWoride to sea water.Most of the experiments were carried out on the spring-spawned larvae while a few confirmatory ones were done in the autumn.