gastrulation differs slightly from birds and mammals. Reptiles exhibit bi-modal gastrulation during embryogenesis and lack a primitive streak. Bi-modal gastrulation is characterized by involution of the cells in the anterior and lateral regions of the blastopore and ingression of the cells of the blastopore plate in the posterior region. Analogies between the blastopore plate and primitive streak suggest the blastopore plate was a precursor to the mammalian and avian primitive streak. See also Embryogenesis Human embryogenesis Hypoblast References Embryology