Cyclic changes in the ovary of the brook stickleback Eucalia inconstans (kirtland)

The ovarian cycle of the brook or 5‐spined stickleback was studied histologically. Specimens were trapped between July, 1965 and August, 1966 in a shallow pond near London, Ontario. Spawning took place from mid‐April to mid‐July, when the water averaged 10°C. In early August, When the water averaged 20–22°C for prolonged periods, spawning ceased and the ovaries contained only oögonia, atretic mature oöcytes and empty follicles. Oögonia and immature oöcytes were found throughout the year but only in the spring did the mature stages begin to appear. Oöcytes for each year's spawning appear to arise mitotically from a residual stock of lightly basophilic oögonia. As the oöcytes mature, their basophilia increases and they acquire a vitelline membrane and follicular layer. The deeply basophilic cytoplasm of the early oöcyte becomes frothy, as oily droplets of primary yolk are laid down. Finally, it becomes filled with eosinophilic secondary yolk in the mature egg. Two types of atresia of developing eggs are common, especially during and after the spawning period. The first involves active phagocytosis of yolky material and hypertrophy of the follicular layer, leading to the formation of what could be called a “corpus luteum.” In the second type of atresia, neither phagocytosis nor hypertrophy was seen and nothing resembling a corpus luteum was formed.