Embryology Lecture

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University of Toronto St. George
Ecology & Evolutionary Biology
De Iuliis

EEB263 Comparative Vertebrate Anatomy Quiz 2 Review Life History Introduction: Life history begins with fertilization, followed by embryonic development, maturation and in some cases senescence, each stage being a prelude to the next. o Embryonic development or ontogeny: Fertilization to birth or hatching. The egg is fertilized and divides into millions of cells, which form the basic structural components of the individual. o Maturation: Time from birth to the point of sexual maturity. Usually involves a growth in size and acquisition of learned skills as well as appearance of anatomical features that distinguish the reproductive- ready adult. Pre-reproductive individuals are called juveniles or immatures. If the juvenile and adult are strikingly different in form, and the change from one to the other is abrupt, then the transformation is termed metamorphosis (i.e. tadpole to frog). o Senescence, or aging: The loss of physical vigor and reproductive ability. This is apparent in humans but rare in wild animals. In fact, senescent animals usually provide an easy meal for ready predators. Few examples of senescence are found in the wild (few species of salmon, and social primates). Early Embryology Introduction: Early in embryonic development, the cells of the embryo (the product of a fertilized egg, from the zygote until the fetal stage) become sorted into three primary germ layers: 1. Ectoderm 2. Endoderm 3. Mesoderm Each layer gives rise to specific regions that form body organs. Structures of two species that pass through closely similar steps of embryonic development can imply homology between these structures. Close homology testifies to the phylogenetic relationship of both species. The youngest stage of the embryo is the fertilized egg, or zygote, which develops subsequently through the morula, blastula, gastrula, and neurula stages. During these early stages, the embryonic area becomes defined from the extraembryonic area that supports the embryo or delivers nutrients but does not become a part of the embryo itself. The delineated embryo first becomes organized into the three basic germ layers and then passes through organogenesis (literally, organ-formation) during which the germ layers differentiate into specific organs. www.notesolution.com
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