Summary When skeleton‐forming cells of a donor sea urchin embryo are transplanted into a host embryo of another species, whose endoderm and mesoderm have previously been removed so that it only possesses ectoderm, a larva may be produced which is a chimaera consisting only of skin and skeleton. The donor skeleton is harmoniously situated in the host larva, thanks to the influence of the latter. But the skeleton affects the host inasmuch as it forces the latter to form larval processes. The skeletal structure is of the donor type. When skeleton‐forming cells are implanted into the whole larva of another species, an intermediate type of skeleton arises, with the exception of the skeletal rods, which occur only in the host form. These are developed exactly as in the host form. When skeleton‐forming cells are implanted into an embryo whose own skeleton‐forming cells have previously been removed, a skeleton develops which at first has the donor structure. Later on the host also supplies skeleton‐forming cells, and the skeleton which has already been formed gradually changes towards that of the host form. Hybrids obtained by cross‐fertilization of the same forms as those which made the chimaeras also have intermediate skeletons. When a species‐hybrid is made (A ♀×B ♂), its skeleton‐forming cells contain only maternal cytoplasm (A), but half maternal (A) and half paternal (B) chromatin. When the skeleton‐forming cells of such a hybrid are implanted into an embryo of the maternal species (A), whose skeleton‐forming cells contain both A cytoplasm and A chromatin alone, a hybrid chimaera is obtained, the skeleton‐forming cells of which contain cytoplasm of the maternal species (A) alone but chromatin of both species in the ratio of 3 A: I B. The skeletons are intermediate, but approach nearer to the maternal type. If the maternal component is weakened by the excision of some skeleton‐forming cells from the host before the implantation, then the skeleton is more definitely intermediate. The formation of a skeletal rod depends on two factors, the presence of the arm‐ectoderrn and of the corresponding skeleton‐forming cells. If the arm‐ectoderm is absent, the corresponding skeletal rod cannot be formed. If the arm‐ectoderm is present, and the skeleton‐forming cells are hybrids between a species which normally possesses a skeletal rod and one which lacks it, then the rod is not formed. Thus the absence of the skeleton‐forming factor is dominant to its presence. A study of normal skeleton formation gives the impression that the skeletogenous cytoplasm of Echinocyamus has a lower viscosity than that of Psammechinus. Protoplasmic viscosity seems to be one of the factors determining the particular structure of the skeleton. The fact that in sea‐urchin larvae with simple skeletons there appear “directed variations” tending towards the type of the more complicated forms is explicable in this manner. It was found to be possible, through the effects of high temperature and chemical substances, to influence larvae of Echinus very consi...