STUDIES ON THE GONADAL DEVELOPMENT OF THE SNAIL CARP,MYLOPHARYNGODON PICEUS

In Hunan,the age of sex maturity of Mylopharyngodon piceus which grows inthe environment of a pond is 5—6 years;the male spawners ripen one year earlierthan the female ones generally.Oocytes and follicular cells are of the same origin,both derived from the oogonia.The oocytes of the pond-cultured Mylopharyngodon piceus can only develop to the period of primary oocytes (phase Ⅳ),hence it isvery necessary to induce maturation division artificially so that meiosis can takeplace and the oocytes can develop from phase Ⅳ to phase Ⅴ.The entire course ofspermatogenesis,from spermatogonium to spermatozoon,can be completed endo-genously in the pond environment.The testes develop to stage Ⅳ in the fifth winterof the first sex cycle;the ovaries develop to stage Ⅲ in the sixth winter of thefirst sex cycle.After this,the testes remain in stage Ⅳ and the ovaries remain instage Ⅲ throughout the winter of every year.The law governing such seasonal chan-ges in the gonads provides a theoretical basis for the selection of the spawners infish-culture.The oocytes develop from phase Ⅲ to phase Ⅳ synchronously.Thehistological structure of the ovaries returns to stage Ⅱ after induced spawning orafter natural degeneration,which testifies that female Mylopharyngodon piceus isthe type of fish that spawns only once a year.According to our research,two different types of yolk are formed in the oocytesof this fish at the age of sex maturity.One type of yolk is intravesicular,whileanother type is extravesicular.If pond management,such as feeding and water-qua-lity regulation,is not appropriate and yolk formation becomes aberrant,then thephenomenon of abortive eggs is bound to occur.This is an important problem infish-culture,the solution of which depends on further researches.