SEX DIFFERENTIATION IN AMPHIBIA

Abstract

THE research which is summarized below was based on the well-known investigations of Witschi, who showed that at an early stage of development the amphibian gonad consists of a cortex and medulla. At a later stage, sex differentiation is marked by an increase of cortex relative to medulla or vice versa. If the former occurs, the undifferentiated germ cells remain in the cortex and become oocytes ; if the latter they tend to migrate into the medulla and become male gametes. Under normal conditions, the relative development of cortex and medulla is determined by the hereditary constitution, that is, by the balance between female- and male-determining genes born on the sex chromosomes and autosomes respectively, from interaction of which one or other of the primary sex 'realizers' is produced. A secondary sex determiner is then secreted by the cortex (cortexin), which initiates oogenesis and simultaneous regression of the medulla, or by the medulla (medullarin), which promotes spermato-genesis and inhibits further development of the cortex. Cortexin and medullarin must be considered as substances distinct from the adult sex hormones¹.