The absolute size of the ovocytes was expressed in mu. The number of ovocytes in a cer- 

 tain area of a histological section was calculated in the following way; by the aid of an ocular-grid 

 all the ovocytes in 5 adjacent fields were counted at a magnification of 80 times . The area of each 

 field, approximately corresponding to the field of the microscope, was exactly 0.81 mm.^. In 

 order to count the ovocytes in all 5 fields, the preparation was shifted by a movable stage. After 

 the determination of the number of ovocytes in different phases of development in an area 5x 0.81 

 mm. , the arithmetical mean was found for each field. The figure found was recalculated for 1 

 mm . . 



Phases in the Development of the Ovocytes . 



Most of the present scales of maturity of fish are based on macroscopical criteria, which 

 do not characterize the maturation processes with sufficient accuracy, neither of the ovocytes nor 

 of the ovary as a whole. More exact results are obtained from histological investigations, which 

 have unfortunately been undertaken in the ovaries of only very few species. For the herring group, 

 the single paper existing is that by Loewe (1903), containing a small amount of data on the matura- 

 tion of the ovocytes . 



If a histological investigation is carried out parallel with the macroscopical description of 

 the gonads, it is possible to work out a scale of maturity, based on objective criteria, reflecting 

 the maturation processes of the gonads. Such a method has been used in the present work. 



Before describing the single stages, we shall shortly give the anatomical structure of an 

 ovary. In the Murmansk herring, the roe is asymmetrical; the left ovary on an average is 2-4% 

 longer than the right one. The walls of the ovary consist of connective tissue and smooth muscles. 

 Transversal ovigerous lamellae protrude from the walls of the ovary (Figure 2). In the upper part 

 of the ovary there is a main blood vein with numerous capillaries to each ovigerous lamella . Each 

 ovary contains a central cavity. On the dorsal side the cavities unite to form an oviduct. 



The growth of the egg cells (ovocytes). Each maturity stage of the ovary Is characterized 

 by a certain complex of egg cells in various stages of growth (in the further report the term phase 

 is used in order to characterize the various epochs in the growth of the egg cells, and stage for the 

 maturity stages of the ovaries). 



The main purpose of this paper is to give a total histological description of the changes in 

 the ovary of the Murmansk herring during the maturation cycle . Cytological details are therefore 

 avoided. The earliest phases in the development are of no special significance for the description 

 of the maturity stages. As will be shown later, they are always present in the ovary. In addition, 

 they do not differ essentially from the descriptions already given in the literature. 



The juvenile phase of the ovocytes (Meien 1939). The ovocytes are of an irregular, angular 

 shape, diameter 43.8-73.0 mu, average 56.94 mu. The cytoplasm forms a thin layer without any 

 visible structure, surrounding the ovocyte. The nuclei of the follicle tissue lie close to the mem- 

 brane, in some distance from each other. 



The cells of the follicle epithelium are of an extended, compressed shape, similar to con- 

 nective tissue. The protoplasm is granular. The nucleus, occupying most of the cell, has a dia- 

 meter of 21.9-36.5 mu, average 27.89 mu. At the periphery of the nucleus numerous nucleoli of 

 an irregular, rounded shape are distributed. The chromosomes lie in a thin, delicate net-work of 

 achromatin. In preserved material, the chromosomes look like thin threads with small granules 

 of chromatin. During the growth of the ovocytes, the nucleus occupies relatively less room, but 

 Increases in absolute size, and the ovocyte is passing over to next phase. 



212 



