For example, haploid human gametes have twenty-three chromosomes when the egg and sperm fuse, the diploid state of forty-six chromosomes is restored to the zygote. Another way to say this is that fertilization restores the diploid number. At fertilization, the gametes combine to produce a zygote with the full number of chromosomes for that particular species. Each gamete is haploid, that is, it contains one-half of the normal number of chromosomes for the species. During fertilization, the genetic material of the sperm and egg are combined. Fertilization is the fusion of the nuclei of the egg and sperm, and the single cell that results from this fusion is called the fertilized egg or zygote (see Figure 2). The motile sperm swims to the egg, pierces its cell membrane and enters the cell. ![]() The process of development begins with the fusion of gametes: egg (ovum) and sperm. Because they delineated the basic concepts of development, Wolff and von Baer are considered founders of modern embryology. These four concepts, referred to as von Baer’s law, are: (1) general features such as appendages appear earlier in the embryo than specialized features such as fingers (2) development proceeds from general to specific characteristics (3) as an embryo develops, it becomes increasingly different from other species and (4) the early embryo of a higher animal is never like an adult of a lower animal, but only similar to the lower animal’s embryo. In 1828, Karl von Baer (1792-1876) described four fundamental concepts of development. This theory, which is termed epigenesis, was largely ignored until 1759 when Kaspar Friedrich Wolff (1733-1794) offered empirical evidence of epigenesis from his detailed studies of chick development. William Harvey (1578-1657) viewed embryological development as a continuing process of remodeling and growth from unspecialized tissues to specialized structures. He postulated that the fertilizing agent was composed of the proteins and fats in the fluid. Although Spallanzani correctly concluded that both egg and seminal fluid were necessary for development, he believed that the sperm seen in the seminal fluid were parasites. If the eggs were combined with the material left on the filter paper, they began to develop. The more highly the seminal fluid was filtered, the fewer eggs developed. However, if the eggs were exposed to filtered seminal fluid, fewer eggs developed. When the eggs were exposed to seminal fluid, they began to develop. He conducted a series of experiments using amphibian eggs and seminal fluid. In 1775, Lazzaro Spallanzani (1729-1799) demonstrated that both egg and seminal fluid were needed to produce a new individual. Anton Leeuwenhoek (1632-1723), a spermist, reported seeing a miniature human (homunculus) in each sperm (see Figure 1). In 1675, Marcello Malpighi (1628-1694), an ovist, reported seeing a miniature chick in the chicken egg. Scientists who believed that the miniature animal was in the egg were called ovists those who believed that the miniature animal was in the sperm were called spermists. Scientific debates over whether the miniature animal was contained in the egg (ovum) or the sperm raged for decades. Preformation means that animals develop from an already existing miniature animal that merely required the right conditions to unfold and grow into a new organism. Prior to the mid-1800s, scientists believed that development was the result of preformation. Aristotle (384-322 B.C.E.), considered the first embryologist, described the growth of a chick embryo from a small dot of tissue to a fully formed bird.Ī scanning electron micrograph of a human embryo at the eight-cell stage (day three). The scientific study of these developmental processes is called embryology. The fertilized egg undergoes cell divisions to increase the number of cells simultaneously, the cells produced differentiate into the organs and organs systems of the fully formed organism. ![]() Multicellular organisms arise through a process that begins with the fertilized egg and ends with a new individual. Reproduction and development are integral factors of life.
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