In general, the mesoderm is responsible for the formation of striated, skeletal, and smooth muscles, bones, cartilage, connective and adipose tissues, the lymphatic and circulatory systems, dermis, the genitourinary system, and the notochord. The notochord or the primitive spine of the animal develops from the differentiation of the cells of the chordamesoderm. The paraxial mesoderm produces the cartilage, skeletal muscles, and the dermis. The cells of the intermediate mesoderm give rise to the kidneys and gonads, while the lateral plate mesoderm differentiates and produces the entirety of the circulatory system including the heart, the various blood cells, blood vessels, spleen, the wall of the gut, and also the wall of the animal body. These four types are formed via cell signaling and cellular interactions between the cells of the endoderm and the ectoderm. The mesoderm further gives rise to four subtypes: Its presence leads to the formation of a body cavity, called the coelom, where the organs develop and grow freely. The presence of this layer is only seen in triploblastic animals. The endoderm, in humans, differentiates and forms distinct recognizable organs within the 6 th week of embryonic development. In less complex animals (protostomes), such as molluscs and earthworms, the blastopore develops into the oral cavity or mouth, but in case of more complex animals (deuterostomes), such as humans and most vertebrates, the blastopore develops into the anal opening. The fate of the blastopore depends on the complexity of the animals. The dorsal part of the blastopore is called the dorsal lip, and the cells around it later give rise to the head region of the animal. The blastocoel is now called the archenteron, and will eventually develop into the digestive tract of the animal.
The blastopore is an opening on the surface of the blastula that opens into the blastocoel. The gastrulation process is characterized by the formation of a blastopore, a dorsal lip, and the archenteron.
Blastopore FormationĪfter implantation, the blastula develops into a gastrula. The trophoblast will eventually give rise to the yolk sac and the amnion, while the inner cell mass will undergo differentiation to form the embryo. As development progresses and the blastula attaches to the uterine lining, it expands and the cells form two distinct structures: the trophoblast and the inner cell mass. The blastula shows cells arranged in a spherical manner called the blastoderm, encircling a fluid-filled cavity called the blastocoel. A blastula is generally formed after the 7 th cell division (cleavage) of the zygote, and consists of around 128 cells.
The zygotes (fertilized egg cells) undergo cell divisions till they form a multicellular ball of cells called the blastula. The three germ layers in chick embryos were first observed and reported by Heinz Christian Pander, a doctoral student at the University of Würzburg, Germany, in 1817.Īnimals, invertebrates, and vertebrates alike, start out as unicellular zygotes.
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