COMPLETELY DIRECT DEVELOPMENT OF ABATUS-CORDATUS, A BROODING SCHIZASTERID (ECHINODERMATA, ECHINOIDEA) FROM KERGUELEN, WITH DESCRIPTION OF PERIGASTRULATION, A HYPOTHETICAL NEW MODE OF GASTRULATION

Development of the brooding schizasterid Abatus cordatus, a subantarct ic echinoid endemic to Kerguelen, is described, Females spawn nonbuoya nt eggs 1300 mu m in diameter, which are fertilized by elongated sperm (head 1 mu m wide and 15 mu m long), The main characteristics of this development an ( 1) incomplete cleavage beginning at the animal pole that becomes holoblastic, giving a filled wrinkled blastula 26 days af ter fertilization: (2) apparent (fate-mapping studies have not been do ne) external migration of mesenchyme cells, in the perivitelline space , from the animal to the vegetal pole during gastrulation while the ar chenteron invaginates; (3) hatching occurring at the end of the gastru lation (65 days after fertilization); (4) differentiation of the vesti bule from a thickening of the oral epidermis as soon as the end of gas trulation is attained; and (5) production of a juvenile directly from the gastrula without any larval stage. The juvenile that leaves the br ood chamber is 2 mm in diameter and about 250 days old. A, cordatus is a true completely direct developer (no larva and no metamorphosis), W e propose to use (1)the term perigastrulation, as a tentative one unti l more definitive studies are available, to describe the hypothetical peculiar movement of cells during gastrulation and (2) the terms of di rect development only for completely direct developing species and abb reviated development for species that have more or less transformed pl utei.