In the summer of 1893, Thomas Hunt Morgan experimented with the eggs of two species of Echinoderm (marine animals typically recognized by their radial symmetry, like sea urchins or sea stars). The first marine organism he studied was Arbacia punctulata, more commonly known as the purple sea urchin. Purple sea urchins were abundant, though their abundance has fluctuated through the years, and were commonly found off the coast near Woods Hole. The second organism, Asterias forbesii, commonly known as the sea star, was also abundantly available. While others at the MBL and elsewhere were merely observing the normal process of development in these marine animals, Thomas Hunt Morgan was beginning to experiment. He was finding ways to understand normal development works by putting living things together in abnormal ways.
In this experiment with Echinoderm eggs, for examples Thomas Hunt Morgan cross-fertilized using the gametes of individuals from two different marine species of the same Echinoderm phylum. In one experiment, he crossed the eggs of Asterias with the sperm of Arbacia. In this experiment, he found that fertilizing sea star eggs with sea urchin sperm resulted in an embryo that would segment, as in normal development, but would do so at a much slower rate than observed in a normally fertilized embryo of either species. Although the crossed embryos would develop until the gastrulation stage, they would typically die shortly thereafter. Morgan noted that Oscar Hertwig and Theodor Boveri had also done similar kinds of experimental work in Europe, notably Boveri’s 1889 work crossing Sphaerechinus eggs with Psammechinus sperm. Like Boveri, Morgan found that the crossed embryos expressed a morphology unlike either the starfish or the sea urchin, an intermediate morphology. On the one hand, this is an example of the introduction of early experimentalism in American biology, and on the other it was a form of both comparison and replication. Morgan experimented with local varieties of organisms that he knew were similar in kind to those that Hertwig and Boveri used from the Mediterranean.