This is the case for the frog’s egg as Roux showed in a classical experiment. Roux destroyed one of the two first cleavage cells of a frog’s egg with a hot needle and found that as a rule the surviving cell developed into only a half-embryo.[127] The frog’s egg consists of two substances, a lighter one which is on top and a heavier one below. Although viscous, the two substances are not too viscous to prevent a flow if the egg is turned upside down. O. Schultze found that if a normal egg is turned upside down in the two-cell stage and held in that position, two full embryos arise, one from each of the two blastomeres. Through the flow of the lighter liquid in the egg upwards the two halves of the protoplasm on top become separated and develop independently into two whole embryos instead of into two half-embryos. In Roux’s experiment this flow of protoplasm was avoided. Morgan showed that if Roux’s experiment is repeated with the modification that the egg is put upside down after the destruction of the one cell, the intact cell will give rise not to a half but to a whole embryo.[128] These experiments prove that each of the first two cleavage cells of the frog’s egg represents one-half of the embryo and that a whole embryo can develop from each half only when a redistribution of material takes place, which in the egg of the frog can be brought about by gravitation since the egg consists of a lighter and a heavier mass. [127] Roux, W., _Virchow’s Archiv_, 1888, cxiv., 113. [128] Morgan, T. H., _Embryology of the Frog_. New York. When, therefore, in the egg of the sea urchin each of the first two blastomeres naturally gives rise to a whole embryo it is due to a greater degree of fluidity of the protoplasm and not to a lack of preformation of the embryo in the cytoplasm. This idea is confirmed by the observations on the egg of _Ctenophores_ whose cytoplasm seems to be more solid than that of most other eggs. Chun found that the isolated blastomere of the first cell division produced a half-larva, possessing only four instead of the eight locomotor plates of the normal animal. It seems that in the egg of molluscs, also, the simple symmetry relations of the body are already preformed. It is well known that there are shells of snails which turn to the right while others turn in the opposite direction. The shells of _Lymnæus_ turn to the right, those of _Planorbis_ to the left. It was observed by Crampton[129], Kofoid, and Conklin that the eggs of right-wound snails do not segment in a symmetrical, but in a spiral, order, and that in left-handed snails the direction of the spiral segmentation is the reverse of that of the segmentation in the right-handed snails. Conklin was able to show that the asymmetrical spiral structure is already preformed in the egg before cleavage. The asymmetry of the body in snails is therefore already preformed in the egg.[130]