The dorsal lip region is called the chorda-mesoderm because it develops normally into mesoderm and notochord. In normal amphibian development the dorsal ectoderm just above the dorsal lip of the blastopore becomes the neural structure. During gastrulation, the potencies of ectoderm become limited from general to specific one. The ectoderm in early gastrula remains in an undifferentiated stage, i.e., it has manifold potencies at that stage. These experimental studies have revealed that the grey crescent material of the egg gives rise to the dorsal lip of the blastopore in early gastrula.īut in the late stage, i.e., when the gastrula transforms into an embryo, the grey crescent materials become localised in the head endoderm of the primitive gut.ĭorsal Lip of Blastopore and its Significance: The vital dye method and other marking devices, such as carbon particle technique and radioactive labelling method have been applied to ascertain the fate of grey crescent during development. Similar operation of removal of grey crescent from the egg prior to cleavage has shown that the process of cleavage remains unaltered, but gastrulation does not occur. The neural tube eventually develops into the brain and spinal cord.The same operation if performed on an egg in which one half gets the entire grey crescent, while the other blastomeres lack the grey crescent material and the two blastomeres when cultured in isolation, the blastomere which is devoid of grey crescent will develop a simple sac of ectoderm containing endoderm, while the blastomere having the grey crescent will form an entire embryo. In time the lips of the folds fuse to form the neural tube. This ectoderm grows up into two longitudinal folds, forming the neural folds stage. The Spemann organizer (mostly mesoderm) will develop into the notochord, which is the precursor of the backbone and induce the ectoderm lying above it to begin to form neural tissue instead of skin. Germ-layer origin of various body tissues Ectoderm a cluster of cells that develops into the Spemann organizer (named after one of the German embryologists who discovered its remarkable inductive properties).Īs gastrulation continues, three distinct "germ layers" are formed:Įach of these will have special roles to play in building the complete animal. This produces an opening (the blastopore) that will be the future anus. The start of gastrulation is marked by the pushing inward ("invagination") of cells in the region of the embryo once occupied by the middle of the gray crescent. All of the activities up to now have been run by gene products (mRNA and proteins) deposited by the mother when she formed the egg. Not until the blastula contains some 4,000 cells is there any transcription of zygote genes. In fact, because the cells of the blastula are so small, the blastula looks just like the original egg to the unaided eye. Figure 14.2.4 Frog Bastulaĭuring this entire process there has been no growth of the embryo. A fluid-filled cavity, the blastocoel, forms within it. By the next day, continued cleavage has produced a hollow ball of thousands of cells called the blastula. However, as cleavage continues, the cells in the animal pole begin dividing more rapidly than those in the vegetal pole and thus become smaller and more numerous. The next few cleavages also proceed in synchrony, producing a 16-cell and then a 32-cell embryo. Figure 14.2.3 Various stages of cleavage in a frog zygote The furrow in the third cleavage runs horizontally but in a plane closer to the animal than to the vegetal pole. The cleavage furrow again runs through the poles but at right angles to the first furrow. The second cleavage forms the 4-cell stage. This divides the egg into two halves forming the 2-cell stage. A furrow appears that runs longitudinally through the poles of the egg, passing through the point at which the sperm entered and bisecting the gray crescent. The first cleavage occurs shortly after the zygote nucleus forms. The zygote nucleus undergoes a series of mitoses, with the resulting daughter nuclei becoming partitioned off, by cytokinesis, in separate, and ever-smaller, cells.
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