Early events in sea urchin metamorphosis, description and analysis.

The larval epithelium of the sea urchin, Lytechinus pictus, consists of squamous cells and bands of columnar epithelial cells bearing cilia. During metamorphosis this tissue undergoes a series of rapid, complex changes. Through the scanning and transmission electron microscope, we describe and analyse these changes.

Immune response in the sea urchin Lytechinus pictus.

The sea urchin shows an immune response to grafted tissue similar to that found in vertebrates. Unrelated animals rejected allografts in about 30 days. Acceptance of allografts was observed for tissue exchanged between some F2 and F3 inbred animals.

Development and life cycle of the parthenogenetically activated sea urchin embryo.

A method is reported for inducing parthenogenetic development in eggs of the sea urchin Lytechinus pictus, a species which previously could not be artificially activated. NH4OH or the calcium ionophore A23187 are used as activating agents followed by hypertonic treatment. The ionophore is superior in activating large numbers of unfertilized eggs, whereas NH4OH produces a larger percent of embryos able to undergo gastrulation.

RATIONALE FOR A UNIVERSAL GENETIC CODE.

A mutation in the genetic code would place new amino acids in certain loci and entirely eliminate amino acids from other loci of practically all proteins in an organism. It is reasonable to postulate that mutations of this kind cannot supplant the original code. The genetic code, once established, would therefore remain invariant.

The isolation of nuclei from eggs and embryos of the sea urchin.

NUCLEI CAN BE ISOLATED FROM THE EGGS AND EMBRYOS OF THE SEA URCHIN BY A FOUR STEP METHOD: (1) washing with dextrose solution until the cells become weak, (2) cytolysis in 0.002 M Mg(++) solution, (3) homogenization of the cytolyzed cells by agitation, and (4) separation of the nuclei from the rest of the homogenate by centrifugation on a layered sucrose gradient. Contamination of the isolated nuclei is less than 1 per cent.