Comparison of Amphibian and Bird Embryos

The differences between amphibian and bird embryonic development can be seen when comparing frog and chick embryos.Though they differ greatly during gastrulation, amphibians and birds complete neurulation in a very similar manner.
Frog embryos progress through gastrulation by a process known as epiboly.During epiboly, the small cells of the animal pole grow down over the larger vegetal pole cells.After this process is finished, a blastopore is formed, and the yolk is tucked inside.Chicks progress through a blastoderm.Since the chick yolk is enormous compared to the frog yolk, epiboly would take far too much energy.The blastoderm begins as a small mound of cells situated on the yolk like a skull cap.As it grows, the blastoderm splits into upper and lower layers:the epiblast and the hyperblast, respectively.The hyperblast grows down around the yolk and becomes a part of the endodermal lining.The epiblast becomes the presumptive tissue of the embryo.Cells begin moving towards the middle of the embryo and form the primitive streak and Henson's node.The primitive streak and Henson's node in the chick embryo is about the equivalent of the blastopore in the frog embryos.
The notochord of the frog comes from the cells in the dorsal lip of the blastopore and takes it's place in the roof of the archenteron.Notochord development begins at Henson's node and progresses down the primitive streak, pushing under the epiblast in the chick embryo.The endoderm in frogs lines the archenteron and the ectoderm is formed from cells on the outside of the gastrula, along with newly formed cells.The mesodermal cells enter through the lateral and ventral blastopore and form itself into a lateral plate.The lateral plate mesoderm splits into an upper somatic mesoderm and a lower splanchnic mesoderm.In between these two layers is the coelom.The area where the splanchnic mesoderm and the endoderm nearly …

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