Sterility and partial sterility in the South African clawed toad following the pricking of the egg.

Bounoure (1934) first described the presence in the egg of the frog Rana temporaria of a special cytoplasmic differentiation, which he termed the 'ger-minal cytoplasm'. He was able to trace the developmental history of this plasm as it moved from its original position just under the cortex of the vegetal pole of the fertilized egg, to its inclusion in a number of cells of the early gastrula endoderm, and also mapped the migration of these cells into the gonad rudi-ments. Bounoure believed that the germinal cytoplasm is the definitive germ-cell determinant, and that its presence in a cell is a prerequisite for the cell to undergo meiosis and differentiate into a gamete. Since Bounoure's discovery, germinal cytoplasm has been described in the eggs of a number of other anuran species and a number of techniques have been used to test Bounoure's determinant hypothesis. Blackler, unpublished) have obtained varying degrees of sterility in tadpoles and metamorphosed frogs derived from eggs whose germinal cytoplasm had been irradiated with ultraviolet light at the earliest stages of development. Attempts to remove the germinal cytoplasm from the egg, and to examine the effects of that removal, have been limited. Nieuwkoop & Suminski (1959) pricked the vegetal pole of the four-cell stage in the South African clawed toad, Xenopus laevis, and thereby provoked the formation of an exudate that presumably contained the germinal cytoplasm (which, according to histological studies, is located immediately under the vegetal pole cortex at this stage). Tadpoles grown from eggs which had been subjected to pricking showed no significant reduction in number of germ cells as compared with the number present in control animals. Nieuwkoop & Suminski concluded that the germinal cytoplasm was not indispensable for the formation of gonocytes, and therefore that the germinal cytoplasm was not a germ-cell determinant. A little later, Fischiarolo (1960) removed cells from the vegetal region of blastulae of the frog Disco