Search results
Results From The WOW.Com Content Network
At the beginning of the ninth week, the embryo is termed a fetus (spelled "foetus" in British English). In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. Human embryology is the study of this development during the first eight weeks after fertilization.
The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the fetal membranes and the placenta. In humans, the embryo is referred to as a fetus in the later stages of prenatal development. The transition from embryo to fetus is arbitrarily defined as occurring 8 weeks after fertilization.
The fetus reaches a length of 20 cm (8 in). Lanugo covers the entire body. Eyebrows and eyelashes appear. Nails appear on fingers and toes. The fetus is more active with increased muscle development. "Quickening" usually occurs (the mother and others can feel the fetus moving). The fetal heartbeat can be heard with a stethoscope.
The embryo becomes embedded in the endometrium in a process called implantation. In most successful pregnancies, the embryo implants 8 to 10 days after ovulation. [ 7 ] The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception".
In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.. The stages are delineated through the development of structures, not by size or the number of days of development, and so the chronology can vary between species, and to a certain extent between embryos.
In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages. [2] [4] The main stages of animal embryonic development are as follows: The zygote undergoes a series of cell divisions (called cleavage) to form a structure called a morula.
This process is called childbirth. The ova, which are the female sex cells, are much larger than the spermatozoon and are normally formed within the ovaries of the female fetus before birth. They are mostly fixed in location within the ovary until their transit to the uterus, and contain nutrients for the later zygote and embryo.
The following are a few of the diseases that can occur and be present during the process. Polyspermy results from multiple sperm fertilizing an egg, leading to an offset number of chromosomes within the embryo. [35] Polyspermy, while physiologically possible in some species of vertebrates and invertebrates, is a lethal condition for the human ...