Sources von Baer, Karl Ernst. Observation and Reflection]. Briggs, Robert, and Thomas King. Darwin, Charles. London: Murray, Developmental Biology. Massachusetts: Sinauer, Haeckel, Ernst. In Jenaische Zeitschrift fur Naturwissenschaft , 8 : 1— Hall, Brian Keith.
Hertwig, Oscar. Jena: Fischer, Hertwig, Oscar, and Richard Hertwig. Die Coelomtheorie. Hertwig, Oscar and Richard. Heft I. Part I. The Anatomy and Histology of the Actinia, with special reference to the neuro-muscular system].
Pubblicazioni della Stazione Zoologica di Napoli 14 : — Huxley, Thomas Henry. Kovalevsky, Aleksandr. Memoires de Academie Imperiale des Sciences de St. Lankester, Edwin Ray. Minot, Charles Sedgwick.
Membrane Structure 4. Membrane Transport 5. Origin of Cells 6. Cell Division 2: Molecular Biology 1. Metabolic Molecules 2. Water 3. Protein 5. Enzymes 6. Cell Respiration 9. Photosynthesis 3: Genetics 1. Genes 2. Chromosomes 3. Meiosis 4. Most argued that the homology of the germ layers across all taxa was impossible because vertebrates and invertebrates do not all have the same organs. Widely recognized evidence to disprove germ layer theory came in , from Hilde Proescholdt Mangold and her doctoral advisor, Hans Spemann , working at the Zoological Institute in Freiburg, Germany.
Mangold transplanted ectoderm harvested from the dorsal lip, the main organizing tissue of the embryo during gastrulation , between donor and host species of newts. The embryos in which she had transplanted the dorsal lip developed an extra body, head, or other nervous system structure. The resultant newts indicated that the transplanted tissue had induced gastrulation and neurulation of surrounding tissue just as it would have in its parent embryo.
Mangold's experiments proved that the germ layers lacked absolutely determined derivatives, a result that dismantled germ layer theory. Additionally, this experiment exemplified a shift in embryological methods that had occurred in the late nineteenth century. Whereas most practitioners had focused on described and compared the anatomy of different embryos, some scientists began to physically manipulate embryos to test hypotheses. These methods helped spur the growth of programs that focused on experimental embryology during the early twentieth century.
Following the work of Mangold and Spemann, other scientists experimented on the three germ layers. Wilhelm His at the University of Basel , in Basel, Switzerland, had discovered neural crest , a derivative of the neuroectoderm, in the chick in His noticed that as the neural tube closed, cells began to migrate away from midline; these cells eventually became called the neural crest.
Twenty years later scientists had begun to look for the derivatives of the neural crest , especially in the head and nervous system. In Julia Platt, a doctoral student studying at Munich University, in Munich, Germany, published the results of her research on the ectodermal, specifically neural crest , derivatives in the head. Based on her studies of Necturus maculosus embryos, a type of aquatic salamander , Platt showed that the cartilage of the branchial arches and parts of the teeth developed from ectoderm.
During the s, researchers studied how neural crest cells migrate. In the s, Nicole Le Douarin, a researcher at the University of Nantes, in Nantes, France, created chimeric quail and chick embryos to track the migration and derivatives of the neural crest. As some researchers investigated the derivatives and movements of neural crest , others examined the interactions of the different germ layers within the embryo.
In Pieter D. Nieuwkoop, at the Royal Netherlands Academy of Arts and Science, in Utrecht, Holland, published an article that addressed the potential of endoderm and ectoderm to induce the formation of their surrounding tissues. Using embryos of the salamander Ambystoma mexicanum , Nieuwkoop showed that when endoderm and ectoderm interact, the endoderm induces mesoderm to form within the adjacent regions of ectoderm.
His experiments also demonstrated that the establishment of the ventral and dorsal regions of the embryo, known as the polarity of the embryo, results from the interactions of the endoderm and ectoderm. Scientists began to research the genetic signals responsible for gastrulation in the mids. In the s, scientists began to show how the signals involved in gastrulation also function in neurulation.
Cells in the basal plate become efferent motor neurons and form the ventral anterior horn of the spinal cord. The two ventral horns bulge ventrally to create ventral median fissure.
The dorsal horns merge to create the dorsal median septum. The lumen of the neural tube becomes the central canal of the spinal cord. The spinal cord extends the entire length of the vertebral canal at week 8 of development. At birth , the conus medullaris extends to the L3 vertebra.
In the adult , the conus medullaris extends to the L1 vertebra. Spinal lumbar punctures must be performed caudally to the conus medullaris to avoid damaging the spinal cord. The dura mater arises from paraxial mesoderm that surrounds the neural tube. The pia mater and arachnoid mater arise from neural crest cells.
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