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Zebrafish Development Fertilization Activated zebrafish embryo (Arrow indicates point of sperm entry) Zygote is formed 200 μm Poleo G A et al. Biol Reprod 2001;65:961-966 ©2001 by Society for the Study of Reproduction Cleavage Stage The fertilized egg undergoes synchronous cleavage This is series of extremely rapid mitotic divisions where the enormous volume of zygote cytoplasm is divided into numerous smaller cells Blastula Stage Blastula stages begin at the eighth division (128 cell stage) Asynchronous cell division begins Cells begin to differentiate Blastula Stage Blastula Stage Fate map Text Gastrulation Gastrulation is a series of extensive cell rearrangements. As a result of gastrulation, the embryo contains three germ layers: the ectoderm, the endoderm, and the mesoderm Gastrulation Segmentation The somites develop (precursor to the vertebrae) , the rudiments of the primary organs become visible, the tail bud becomes more prominent and the embryo elongates Pharyngula The embryo is most evidently now a bilaterally organized creature The nervous system is hollow and expanded anteriorly The brain is now sculptured into 5 lobes Hatching Embryo continues to grow at about the same rate Rapidly developing rudiments of the pectoral fins, the jaws, and the gills One Day Embryo Two Day Embryos Three Day Embryos Newly Hatched 3 Day Zebrafish Adult Zebrafish