By the end of this section, you will be able to:
Juan Gaertner Reproduction and development are integral factors of life. Multicellular organisms arise through a process that begins with the fertilized egg and ends with a new individual. 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.
The scientific study of these developmental processes is called embryology. Early Ideas in Embryology Prior to the mids, scientists believed that development was the result of preformation. 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.
Scientific debates over whether the miniature animal was contained in the egg ovum or the sperm raged for decades. 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.
InMarcello Malpighi —an ovist, reported seeing a miniature chick in the chicken egg. Anton Leeuwenhoek —a spermist, reported seeing a miniature human homunculus in each sperm see Figure 1.
InLazzaro Spallanzani — demonstrated that both egg and seminal fluid were needed to produce a new individual. He conducted a series of experiments using amphibian eggs and seminal fluid. When the eggs were exposed to seminal fluid, they began to develop. However, if the eggs were exposed to filtered seminal fluid, fewer eggs developed.
The more highly the seminal fluid was filtered, the fewer eggs developed. If the eggs were combined with the material left on the filter paper, they began to develop. 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.
He postulated that the fertilizing agent was composed of the proteins and fats in the fluid. A scanning electron micrograph of a human embryo at the eight-cell stage day three. William Harvey — viewed embryological development as a continuing process of remodeling and growth from unspecialized tissues to specialized structures.
This theory, which is termed epigenesis, was largely ignored until when Kaspar Friedrich Wolff — offered empirical evidence of epigenesis from his detailed studies of chick development.
InKarl von Baer — described four fundamental concepts of development. These four concepts, referred to as von Baer's law, are: Because they delineated the basic concepts of development, Wolff and von Baer are considered founders of modern embryology.
Fertilization The process of development begins with the fusion of gametes: The motile sperm swims to the egg, pierces its cell membrane and enters the cell. 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.
During fertilization, the genetic material of the sperm and egg are combined.ANATOMY OF THE HEAD AND NECK (basic course for students of Dentistry) Chisinau The head and neck comprise a highly specialized region of the body.
The structures contained within this region are closely interrelated because they are compacted into a small, complicated area.
The Indiana University School of Medicine Division of Continuing Medical Education is a large and robust provider of CME with activities ranging from large multi-day conferences to online modules. Gray, Henry.
Anatomy of the Human Body. XII. Surface Anatomy and Surface Markings. 2. Surface Markings of Special Regions of the Head and Neck.
Essay # 2. Morphology of Frog: The body of frog is divisible into two parts only: the head and the trunk. The neck and tail are not present.
The anteriorly placed mouth is a wide transverse opening. In a remote corner of El Salvador, investigators uncovered the remains of a horrible crime — a crime that Washington had long denied.
The villagers of El Mozote had the misfortune to find themselves in the path of the Salvadoran Army's anti-Communist crusade.
As a follow-up to Tuesday’s post about the majority-minority public schools in Oslo, the following brief account reports the latest statistics on the cultural enrichment of schools in Austria. Vienna is the most fully enriched location, and seems to be in roughly the same situation as Oslo. Many thanks to Hermes for the translation from benjaminpohle.com