Regional specification during embryogenesis in the articulate brachiopod Terebratalia.

The timing of regional specification during embryogenesis in Terebratalia transversa has been examined by isolating animal, vegetal, and lateral halves at developmental stages ranging from prior to fertilization through late gastrula. Animal halves isolated from prior to fertilization through late blastula stages did not gastrulate and did not form endoderm. When these animal halves were isolated from prior to fertilization through early blastula, they formed only ciliated vesicles; however, similar halves from late blastulae frequently formed the ectodermal components of the three larval lobes. Vegetal halves isolated prior to fertilization through late blastula gastrulated, formed endoderm, and differentiated most larval structures; however, comparable isolates from late gastrulae showed a marked decline in the ability to form anterior larval structures. Animal halves isolated at this stage showed a corresponding decline in the ability to form posterior larval structures. These results indicate that during oogenesis, regional differences arise so that the animal half of the oocyte has a much greater ability to form anterior ectodermal structures than do other regions of the oocyte, while only the vegetal region has the capacity to gastrulate. Inductive signals produced by the vegetal region of the embryo act on the animal half to give it the capacity to form ectodermal components of the three larval lobes. This vegetal-inductive influence has played a major role in regional specification by the late blastula stage of development. Pairs of lateral halves isolated from the same oocyte or embryo from prior to fertilization through late blastula frequently gastrulate, form endoderm, and proceed to form all or most of the larval lobes. The late blastula is the first developmental stage where there is a marked decline in the ability of pairs of lateral halves to differentiate larval structures and is the first stage where pairs of halves resembling dorsal and ventral halves appear. This suggests that the axis of bilateral symmetry is also specified by the late blastula stage of development.