THE EARLY DEVELOPMENT OF COREIUS HETERODON AND COREIUS GUICHENOTI

Coreius heterodon and C. guichenoti, the two of the total three species belonging to the genus Corcius, are most famous fishes of the upper and middle reaches of the Changjiang Valley. The most conspicuous external features comparing with those of other cyprinids of the Changjiang River are smaller eyes and rather longer maxillary barbels.The successive developmental stages of the early development of these two fishes were examined under natural water tempertures, ranging from 18.2 to 32.8℃. The newly cast eggs were collected during the summer seasons of the years 1961—1966, 1973, 1976—1978 and 1981 in the very spawning area of the main streams of the Changjiang River and one of its main tributaries, the Hanjiang. Although the rivers were obstructed at Danjiangkou and Gezhouba by the construction of dams, yet most of the spawning grounds are still functional and even extended both to the main reaches below the dams and to the upper part of the Danjiangkou reservoir.The morphological features of 49 stages of the embryonic and postembryonic development of these fishes are rather similar with each other. However, there are also some obvious differences in a few stages. The formation of proembryo of C. guichenoti during embryonic development occurs in middle gastrula, while it arises in late gastrula for C. heterodon. The first appearance of the muscle segments is prior to the formation of eye vesicle in C. guichenoti, but they come out in reverse order in the other species. Furthermore, the emergence of the barbel bud is earlier, the pectoral fin is shorter and the eye is a little larger in C. heterodon than that of C. guichenoti throughout the period of post-embryonic development.