ON THE EMBRYONIC DEVELOPMENT OF THE SEA URCHIN ALLOCENTROTUS FRAGILIS

¹ Hopkins Marine Station of Stanford University, Pacific Grove, California

1. Ripe eggs and sperm of Allocentrotus fragilis were obtained during February and March, from animals taken at depths of 80 to 100 fathoms.

2. Fertilization was approximately 100 per cent, and development was normal at a temperature of 15° C. or lower.

3. The rate of development in Allocentrotus fragilis is identical to that of the two species of Strongylocentrotus at this locality.

4. The pluteus has a characteristic form which distinguishes it from those of S. purpuratus and S. franciscanus.

5. Cross-fertilization between Allocentrotus fragilis and the two forms of Strongylocentrotus was accomplished, and in the progeny of A. fragilis x S. franciscanus the development of paternal characters was clearly shown.

6. As to the cause of spring spawning, increase in light and possible increase in temperature are not factors, but Thorson's idea of the occurrence of algal extracts of the plankton in early spring is suggested as a possible factor. There is also the possibility of an innate periodicity characteristic of the species.