CYTOLOGICAL STUDY OF FERTILIZATION IN A FRESH WATER TELEOST, ARISTICHTHYS NOBILIS R.

Because of the difficulties of obtaining oocytes of optimal degree of maturation,only few investigations of fertilization in Chinese fresh water teleosts have been reported. It is therefore of great interest to study the cytological behavior of fertilization in Aristichthys nobilis. The main cytological features that we have observed are summarized as follows: 1. Fertilization was found to take place in the egg at the metaphase of the secondmaturation division and to be monospermic. Micropyle and micropylar cell were foundin the aminal pole of the ovulated oocyte (Fig. 1-4). Without the small opening ofmicropyle and the canal leading from micropyle cell to the egg proper, the penetrationof spermatozoon seems impossible. 2. Rows of cortical alveoli were observed to be lying in the cortex of mature oocyteclose to the plasma membrane (Fig. 1c). A successive breakdown of cortical alveoli lasting 10 minutes was induced after artificial fertilization (26-27℃). In the process ofbreakdown, each alveolus formed an aperture at the point of contact with the plasmamembrane. Through the aperture the content of alveolus was emptied into the perivitelline space beneath the fertilization membrane (Fig. 5). The cortical reaction hasbeen regarded as a general phenomenon accompanying fertilization and parthenogenesis.The contents of cortical alveoli may differ in different species of animals, but thephysiological significance of the cortical reaction is the same, as proposed by the "Epuration" theory of Prof. Bataillon and Prof. Tchon-Su. 3. The expansion of sperm aster, the migration of sperm head, the completion ofthe second meiotic division of the oocyte and the climination of the second polar bodywere all accomplished at the end of 5-10 minutes after fertilization(Fig. 6-8). Theformation of male and female pronuclei was found about 5 minutes later (Fig. 9).Subsequently, the conjugation of male and female pronuclei (Fig. 10) and the formationof the double spindle of the first cleavage (Fig. 11) were observed. The first cleavagefurrow of the blastodise appeared at about 30 minutes after artificial insemination.The intervals between the successive divisions were found to be about 10 minutes. A lot of common features were observed during the fertilization processes of Aris-tichthys and other species of bony fishes belonging to Cyprinidae (e. g. Carassius auratus,Megalobrama terminalis, Hypophthalmichthys molitrix. Ctenopharyngodon idella, etc.).However, there are species-specific features, so that the hybridization among them mayor may not be realized successfully.