Experiments on the Developmental Potencies of Blastoderms and Fragments of Teleostean Eggs separated latitudinally

The developmental potencies of blastoderms and fragments separated latitudinally during the cleavage stages of the egg of the goldfish have been studied. At the 1- and 2-cell stages hyperblastuJæ represent the highest differentiation attained by fragments separated at or above the equator. Fragments of the same size isolated at the 4- to 16-cell stage may give rise to perfect or nearly perfect embryos. Blastoderms removed before the 8-cell stage form hyperblastulæ as their highest differentiation. Those removed at the 8-cell stage, or later, show differentiation of embryonic structure or give rise to typical embryos. The results are explained by the existence in the periblast of a substance necessary for gastrulation and for the organization of the embryo. This substance is located unilaterally below the equator before, or just after, the first cleavage; it flows up and passes through the latter during, or after, the 2-cell stage, and finally enters the blastoderm during, or possibly after, the 4-cell stage. In hyperblastulæ, epithelium and abortively self-differentiated nervous, notochordal and mesodermal tissues arise in the absence of gastrulation. Among the imperfect embryos there is a gradation of abnormalities ranging from embryos with a defective fore-brain to trunk-like and tail-like bodies: Considerable differentiation of tissues and organs takes place, though all types of structure do not necessarily occur, in a single individual, or are arranged in proper relationship to each other. A nerve cord may be developed independently in the absence of a notochord. Regularly arranged mesodermal rings are often formed unassoeiated with nervous tissue. Defects in the fore-brain may result in failure of the development of optic cups. In spite of the absence of the latter, the lens may be formed, perhaps by self-differentiation. The presence of considerable powers of self-differentiation of the embryonic cells of teleostean eggs, as well as the organizing action exerted by the organization centre, suggest that the normal process of development in teleosts is the result of the interaction between the two systems.