5/18/2023 0 Comments Blastopore.This results in the creation of four identical blastomeres - separate cells now forming the embryo. The second cleavage also cuts through the gray crescent, although always running perpendicularly to the first one. The first cleavage runs across the animal-vegetal axis, dividing the gray crescent into two parts. The cleavage (cell division) of a frog’s embryo is complete and uneven, because most of the yolk is gathered in the vegetal region. Its establishment determines the location of the dorsal and ventral (up-down) axis, as well as of the anterior and posterior (front-back) axis and the dextro-sinistral (left-right) axis of the embryo. This results in the creation of the gray crescent. A relatively short time after fertilization, the cortical cytoplasm (located just beneath the cell membrane) rotates by 30 degrees. Two blocks - defensive mechanisms meant to prevent polyspermy - occur: the fast block and the slow block. ![]() The sperm cell enters the oocyte in the region of the animal pole. The pigment is composed of light-absorbing melanin. The animal pole of the cell contains pigment cells, whereas the vegetal pole (the yolk) contains most of the nutritive material. The oocyte in these frog species is a polarized cell - it has specified axes and poles. The embryonic development of tailless amphibians is presented below using the African clawed frog ( Xenopus laevis) and the northern leopard frog ( Rana pipiens) as examples. One of the most basic criteria of such development is independence from a water habitat.Īmphibians were the earliest animals to adapt themselves to a mixed environment containing both water and dry land. Main article: Embryogenesis Early stages of embryogenesis of tailless amphibians Įmbryogenesis in living creatures occurs in different ways depending on class and species.
0 Comments
Leave a Reply. |