Summary.1The development of egg‐fragments, isolated blastomeres, and fused eggs of the goldfish (Carassius) is described in the present paper.2Fragments derived from eggs about four to forty‐three minutes after fertilization give rise to (i.) perfect embryos of half size, (ii.) abnormal embryos of all degrees, and (iii.) rounded or oval vesicles without any histogenesis. The nearer the time of operation to the time of fertilization, the more perfect the embryos obtained.3Blastomeres were separated following the first or second cleavage plane in the 2‐, 4‐, and 8‐cell stages. The development of these isolated portions varies from the formation of two typical embryos of half size to two vesicles without any histological differentiation. No constant relation between the cleavage planes along which the eggs were separated and the subsequent development of the blastomeres was observed.4Two eggs were fused in the stage of two to thirty‐two cells. The double egg may produce a single perfect embryo of double size or twin embryos either normal or abnormal in structure.5The segmentation of the egg‐fragments is the same as that of the whole egg, while that of the isolated blastomeres is partial, as though still a part of the whole. The divisions are not always equal. A normal embryo, however, may develop from a fragment of unequal division; conversely, a fragment of typical division may give rise to an abnormal embryo.6The abnormal embryos derived from the egg‐fragments and isolated blastomeres are quite variable in form and structure. In some cases the head is well formed while the trunk is abnormal; in other cases the trunk is typical but the head is missing; in still other cases only a tail‐like body is formed.7The nerve‐cord is not always associated with the notochord; a well‐elongated neural tube may be formed without a notochord. No evidence is found that the latter can change the differentiation of epithelial cells with which it is in contact.8In some vesicles developed from the egg‐fragments and isolated blastomeres abortive independent differentiation of somite and nervous tissue is found. A feeble determination of “organ‐forming substances” occurs, therefore, in the very early stages of development.9It is supposed that in the egg of the goldfish there exists some centre comparable to the amphibian grey crescent, from which the organizer region later arises. The normal process of development is based in part on the action of the organizing centre and in part on the self