Experimental conditions that allow "normal" development of starfish eggs stripped of the fertilization membrane are reported in this paper. Four kinds of intercellular relation are distinguished during the pre-hatching stages of these eggs.Cells from 2-to 8-cell stages are hardly related to each other, while those from 16-to 128-cell stages are bound loosely together. After the 8th division (about 5.5 hr after insemination at 21°C) cells adhere closely and cooperate with each other to perform morphogenetic movement of "blastulation". This relation is taken over by that of a true multicellular system at about 10 hr after insemination. Closely after this, the function of cilia carries the embryo away from the substratum.The pre-hatching stages of development, in the author's opinion, could be described essentially as a process of constructing multicellular modes of life (swimming blastula as the simplest of these) from a single self-reliant diploid cell (fertilized egg). This process, therefore, should contain various items of information concerning how the descendents of a self-reliant cell lose this characteristic and grow strongly cooperative with each other. Studies of cell behaviour are necessary, in order t o comprehend this process.Although these stages of development have long been studied as a problem of cell division and, therefore, as a matter of allocation of egg cytoplasm and reduction of cell size, some investigations are available in which the cells were comprehended as intact and actively roleplaying beings. These are observations on the fate of echinoderm embryos stripped of the fertilization membrane (3, 8, 9); on the waves of cleavage passing from cell t o cell in frog embryos (1 2, 13); and cinemicrographic studies of early development of Fundulus (1 5 ) . The pre-eminent method for producing allophenic mice (6,7,14) could also be counted among this line of investigations.Most of these investigations, however, have been prevented from progressing much further by various difficulties. In addition t o the lack of general ideas t o guide the investigators to an understanding of these complex and 'closely integrated processes, some of the main difficulties were 1) the presence of tough fertilization membrane confining the cells into a small space and, thereby, concealing the behaviour of individual cells, 2) the presence of a second membrane, e.g. hyaline membrane (sea urchin) and surface coat (frog), making necessary the fise of various artificial condition, such as cation-free media, 3) the huge size and fragile nature of the cells (frog), and 4) the complexity of the developmental mechanism as a whole (mouse).The pre-hatching stages of the starfish, however, are free of most of these difficulties so that they make a much better material for the purpose of studying cells in the very early stages of development. Because of the lack of a functional hyaline membrane (Figs.la-ld), the * This paper is dedicated to Prof. Emer. Katsuma Dan for the celebration of his 70th birthday.