Investigations of the germ-plasm in relation to nuclear transplantation.

THE origin of the germ-cells has been extensively investigated in both invertebrates and vertebrates. In invertebrates it has been traced to the early cleavage of the zygote, e.g. in such forms as Ascaris megalocephala (Boveri, 1887) and Sciara (Metz, 1938). Within the vertebrate group the cases in which primordial germ-cells have been detected in cleavage stages are very few. Eigenmann (1891) identified gonocytes of Micrometrus aggregates in the late gastrula stage on the basis of their large size and the uniform distribution of their chromatin. The first clear demonstration of vertebrate primordial germ-cells appearing in a stage as early as the blastula was made in the European frog, Rana temporaria (Bounoure, 1934). In this case germ-cells were found to be conspicuous because of a stainable cytoplasmic element, the germ-plasm. This germ-plasm first appears shortly after fertilization in the form of islets concentrated in the vegetal pole region of the egg (Bounoure, 1934,1939,1954). In these investigations Bounoure traced the plasm-bearing cells to the blastula stage, at which time the germplasm is incorporated into cells near or on the floor of the blastocoele. In later stages of embryonic development these primordial germ-cells were traced to the genital folds. That these cells are directly ancestral to the mature germ-cells was indicated by subjecting the vegetal pole of R. temporaria eggs to ultraviolet irradiation shortly after fertilization (Bounoure, 1939, 1954). Eggs treated in this way developed into metamorphosed animals whose gonads, reduced in size, possessed only a small quantity of germ-cells, and in a few cases were devoid of germ-cells. More recently, Blackler (1958) has confirmed Bounoure's studies in R. temporaria and has also made comparable studies on other Anuran species. In Bufo bufo and Xenopus laevis, the presence of germinal cytoplasm corresponded, in general, to that of R. temporaria; however, no germinal cytoplasm was seen in gastrulae or neurulae of R. esculenta. Grafting experiments (Blackler, 1960) have demonstrated that the 'germ-cell region' of X. laevis