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![]() The mesoderm is the third germ layer it forms between the endoderm and ectoderm in triploblasts. The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. The endoderm gives rise to the lining of the digestive tract (including the stomach, intestines, liver, and pancreas), as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures. These germ layers are programmed to develop into certain tissue types, organs, and organ systems during a process called organogenesis.Įach of the three germ layers is programmed to give rise to particular body tissues and organs. Different cell layers (called germ layers) are formed during gastrulation. This leads to the formation of the next developmental stage, the gastrula, in which the future digestive cavity is formed. Next, the blastula undergoes further cell division and cellular rearrangement during a process called gastrulation. After further cell division and rearrangement of existing cells, a 6–32-celled hollow structure called a blastula is formed. Three cell divisions transform the single-celled zygote into an eight-celled structure. The process of animal development begins with the cleavage, or series of mitotic cell divisions, of the zygote (Figure 2). Regardless of whether a species undergoes complete or incomplete metamorphosis, the series of developmental stages of the embryo remains largely the same for most members of the animal kingdom. For the latter, the young and the adult may have different diets, limiting competition for food between them. Other animals, such as some insects, undergo complete metamorphosis where individuals enter one or more larval stages that may in differ in structure and function from the adult (Figure 1). Some animals, such as grasshoppers, undergo incomplete metamorphosis, in which the young resemble the adult. During this process, animal tissues begin to specialize and organize into organs and organ systems, determining their future morphology and physiology. Snodgrass, USDA)Īfter fertilization, a series of developmental stages occur during which primary germ layers are established and reorganize to form an embryo. ![]() (b) The butterfly undergoes complete metamorphosis. (a) The grasshopper undergoes incomplete metamorphosis. However, for animals that are limited in their capacity to attract mates, asexual reproduction can ensure genetic propagation.įigure 1. These types of asexual reproduction produce genetically identical offspring, which is disadvantageous from the perspective of evolutionary adaptability because of the potential buildup of deleterious mutations. This type of parthenogenesis is called haplodiploidy. In contrast, a form of asexual reproduction found in certain insects and vertebrates is called parthenogenesis (or “virgin beginning”), where unfertilized eggs can develop into new male offspring. The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, where part of a parent individual can separate and grow into a new individual. Some animal species-including sea stars and sea anemones, as well as some insects, reptiles, and fish-are capable of asexual reproduction. This process produces a diploid fertilized egg called a zygote. Typically, the small, motile male sperm fertilizes the much larger, sessile female egg. Describe the roles that Hox genes play in developmentĭuring sexual reproduction, the haploid gametes of the male and female individuals of a species combine in a process called fertilization.Compare and contrast the embryonic development of protostomes and deuterostomes.Explain the processes of animal reproduction and embryonic development.
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