The blastocoel probably serves two major functions in frog embryos: (1) it permits cell migration during gastrulation, and (2) it prevents the cells beneath it from interacting prematurely with the cells above it. (B) 8-cell embryo showing a small blastocoel (arrow) at the junction of the three cleavagefurrows. (A) First cleavage furrow, showing a small cleft, which later develops into the blastocoel. As cleavage progresses, the animal region becomes packed with numerous small cells, while the vegetal region contains only a relatively small number of large, yolk-laden macromeres.įormation of the blastocoel in a frog egg. This unequal holoblastic cleavage establishes two major embryonic regions: a rapidly dividing region of micromeres near the animal pole and a more slowly dividing vegetal macromere area ( Figure 10.2C Figure 2.2E). It divides the frog embryo into four small animal blastomeres (micromeres) and four large blastomeres (macromeres) in the vegetal region. However, because of the vegetally placed yolk, this cleavage furrow in amphibian eggs is not actually at the equator, but is displaced toward the animal pole. The third cleavage, as expected, is equatorial. This cleavage is at right angles to the first one and is also meridional. Figure 10.2B shows that while the first cleavage furrow is still cleaving the yolky cytoplasm of the vegetal hemisphere, the second cleavage has already started near the animal pole. One can see the difference in the furrow between the animal and the vegetal hemispheres. (A, B) Because the vegetal yolk impedes cleavage, the second division begins in the animal region of the egg before the first division has divided (more.)įigure 10.2A is a scanning electron micrograph showing the first cleavage in a frog egg. Cleavage furrows, designated by Roman numerals, are numbered in order of appearance. In epibolic gastrula, there is no invagination, but the endo-mesodermal cells become internalized through overgrowth by the ectodermal cells the endoderm differentiates into the gut, and mouth and anus develop from ectodermal invaginations, stomodaeum and proctodaeum, respectively.Cleavage of a frog egg.In invagination gastrula, the gastrula develops from a blastula through invagination of the endoderm forming the archenteron with the blastopore.Deuterostomy is a condition in which the blastopore was retained as the bilaterian anus, the mouth developing as a secondary opening.Protostomy is a condition in which the blastopore was retained as the bilaterian mouth, the anus developing as a secondary opening.Amphistomy is a condition in which the tubular gut evolved from the sack-shaped gut through lateral blastopore closure, leaving mouth and anus.The tubular gut of the bilateral animals evolved from the sack-shaped gut of the common ancestor.The latest common ancestor of the eumetazoans (ctenophores and placozoans are not discussed here) was a gastrula-like organism, with a sac-shaped gut with a blastopore. It is concluded that the tubular gut with mouth and anus most probably evolved through amphistomy. fate of the actual blastoporal opening fate of the tissues surrounding the blastoporal opening, studied both through cell-lineage and gene expression morphology and embryology of the central nervous systems and morphology of larval ciliary bands according to the trochaea theory. A recent review has discussed the most informative characters related to the blastopore fates, viz. Three theories for the evolution of the tubular gut prevail: (1) Protostomy in which the blastopore should become the mouth and the anus develop secondarily, (2) Deuterostomy in which the blastopore should become the anus and the mouth develop secondarily and (3) Amphistomy in which the blastopore should become divided into mouth and anus through fusion of the lateral blastopore lips. Cell-lineage studies show that gastrulation through epiboly and invagination follow similar patterns with the cells of the blastopore rim bordering the cells which give rise to endo-mesoderm. The bilaterian tubular gut with mouth and anus is generally believed to have evolved from the sack-shaped gut of a gastrula-like organism.
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