![]() In other words, it consists of parts which have been classified as ectoderm. The external lining consists of presumptive epidermis and presumptive nervous system. In this stage there is still a space between the external and internal walls representing the remnants of what was the blastocoele of the blastula. The external and internal epithelial layers are continuous with each other over the rim of the cup-shaped embryo. The cup has a double wall, an external one and an internal one, the latter lining the newly formed cavity. Gastrulation is initiated when the blastoderm at the vegetal pole becomes flat and subsequently bends inward, so that the whole embryo, instead of being spherical, becomes converted into a cup-shaped structure with a large cavity in open communication with the exterior on the side that was originally the vegetal pole of the embryo. The necessity of displacement of the parts so as to put them in the positions where they are situated in the adult animal is amply evident. The various cytoplasmic substances present in the egg suffer no appreciable displacement during cleavage, and the fate map presented can equally well be applied to the blastula. The cells at the vegetal pole are somewhat larger than at the animal pole, and the blastoderm therefore is thicker. There is also a striking similarity to the distribution of different kinds of cytoplasm in the ascidian embryo.Īs a result of cleavage in Amphioxus, a ‘blastula is formed which has a large blastocoele and a blastoderm consisting of a single layer of columnar cells. The similarity between this fate map and that of the amphibians is practically complete. ![]() On the opposite side a zone giving rise to the notochord could be detected, and above that a crescentic area which develops into the nervous system. It was found that the presumptive mesodermal area is not restricted to one half of the egg only but reaches farther around the equator. More recently, the method of local vital staining has been applied to the study of Amphioxus development. The crescent of basophilic cytoplasm is the material which gives rise to the muscles and the lining of the body cavity and thus represents a mesodermal area. The granular cytoplasm, which takes up the region around the vegetal pole of the blastula, develops into the lining of the alimentary canal. The fate of the three regions is the following- The clear cytoplasm that later becomes the animal hemisphere of the blastula develops mainly into skin epidermis. They are the smallest cells in the blastula, even smaller than the animal cells, and they are rather loosely packed, the external surfaces bulging out, as is usually found in the earlier cleavage stages. The cells containing the basophilic cytoplasm are clearly discernible even as to shape. The cells are columnar and form a very closely packed columnar epithelium. The animal hemisphere is made up of cells containing the clear cytoplasm. The vegetal material is now contained in a number of rather large cells taking up the position on and around the vegetal pole of the blastula. The distinctions which could be traced in the cytoplasm of the egg now become accentuated by further distinctions in the size and shape of the blastomeres. The mass of cytoplasm of this kind has a crescentic shape, the attenuated ends of the crescent being drawn out along the equator of the egg about halfway around.ĭuring the period of cleavage the three regions become subdivided into blastomeres without the cytoplasmic substances having been displaced to any great extent. On one side of the egg, in a position roughly corresponding to the marginal zone of the amphibian egg, there is a special type of cytoplasm it does not contain much yolk, but it can be distinguished from the animal cytoplasm by its ability to be deeply stained by basic dyes. The animal hemisphere of the egg consists of cytoplasm that has less yolk and is consequently more transparent. Near the vegetal pole a mass of cytoplasm is found which contains the greatest amount of yolk (although yolk in this case is not abundant and the yolk granules are relatively very small). In Amphioxus, there are differences in the various regions of the egg cytoplasm which permitted Conklin (1932) to trace these regions into the later stages of development and thus to reconstruct a fate map, at least in rough outlines.Īt the beginning of cleavage three regions can be distinguished in the Amphioxus egg. In this article we will discuss about the process of gastrulation in amphioxus and amphibians.
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