Department of Mathematics, University of British Columbia, Vancouver, British Columbia, Canada.|Department of Mathematics, University of British Columbia, Vancouver, British Columbia, Canada.|Department of Mathematics, University of British Columbia, Vancouver, British Columbia, Canada.|Department of Biology, Simon Fraser University, Burnaby, British Columbia, Canada.|Division of Developmental Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland, United States of America.|Division of Developmental Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland, United States of America.|Department of Mathematics, University of British Columbia, Vancouver, British Columbia, Canada.