they become localized in the ventrolateral region of the blastocoel. In birds Similar to mammals, fertilization of the avian ovum occurs in the oviduct. From there the blastodisc, a small cluster of cells in the animal pole of the egg, then undergoes discoidal meroblastic cleavage. The blastoderm develops into the epiblast and hypoblast and it is between these layers that the blastocoel will form. The shape and formation of the avian blastodisc differs from amphibian, fish, and echinoderm blastulas, but the overall spatial relationship of the blastocoel remains the same. Formation of primitive streak The avian blastocoel is important during the development of the primitive streak. The ingression of the endodermal precursor cells form the epiblast into the blastocoel and the migration of lateral cells of the posterior epiblast towards the center form the early primitive streak. As these cells converge inward, a depression forms called the primitive groove and functions as an opening through which cells travel into the blastocoel. As cells migrate into the blastocoel, they undergo an epithelial-to-mesenchymal transformation. Zebrafish: a special case Unlike amphibian, echinoderm, mammalian, and avian embryos, zebrafish do not have a defined blastocoel. Rather, they have small, irregular extracellular spaces that are formed between the cells of the blastodisc sitting atop the yolk. References Further reading Dorlands Staff (2004). "blastocoel [distionary entry]". Dorland's Illustrated Medical Dictionary (online). Amsterdam, NDE: Elsevier-Saunders. Retrieved 30 January 2016. "blastocoel...[blaso- + -coele] the fluid-filled cavity of the mass of cells (blastula) produced by cleavage of fertilized ovum. Sometimes spelled...[c]alled...'Also' blastocoelic ...pertaining to the blastocoele."; Dorlands.com Animal developmental biology