The Oxford English Dictionary lists "octopuses", "octopi", and "octopodes", in that order, reflecting frequency of use, calling "octopodes" rare and noting that "octopi" is based on a misunderstanding. The giant Pacific octopus Enteroctopus dofleini is often cited as the largest known octopus species. The head includes the mouth and brain.
Fetal development is the longest and clinically the most important phase of this process. Early Development Early human development may be summarized by the following four phases: This stage takes place during the first week of human embryology.
Fertilization occurs when the cell membrane of a sperm fuses with the cell membrane of the oocyte egginjecting its nucleus. The egg then undergoes its second division of meiosisand the resulting haploid nucleus fuses with the haploid sperm nucleus to re-form the diploid number of chromosomes.
These events occur in the oviduct, or fallopian tube. The fertilized egg, now termed a "zygote," continues to move down the oviduct to the uterus, where it lodges in the wall.
After the conceptus is implanted in the uterine wall, the cells that will form the embryo proper divide and organize themselves into a bilaminar two-layered disc. This disc is surrounded by an outer ring of cells, the trophoblast, which does not contribute to the new organism's tissues.
After implantation, trophoblast cells multiply rapidly and invade the endometrium uterine wall. Together, the trophoblast and endometrium form the placenta, through which all the nutrition for the developing embryo will pass.
This rich mass of tissue is filled with blood vessels, allowing rapid exchange of nutrients and waste. Another group of cells separates from the developing embryo near this time, and these cells also do not form part of the new organism. Instead, they develop into the amnion, the membrane that will surround the fetus to form the embryonic sac.
This fluid-filled sac helps to cushion the fetus during later development. This phase begins during the second week of development.
During the third week, the embryo undergoes the process of gastrulation, forming a trilaminar three-layered disc. Gastrulation establishes the three germ layers—the endoderm, ectoderm, and centrally placed mesoderm—all of which will give rise to the various organ systems.
Mesoderm also combines with trophoblast tissue to form the umbilical cord, which transports nutrients and wastes between the fetal circulation and the placenta.
At this point, the developing human enters the actual embryonic phase, which lasts from the third week through the eighth week after conception.
The organ systems differentiate at greatly varying rates during this phase. For example, the circulatory system is largely functional at the end of this period, whereas the nervous system is still engaged in massive cell division and only beginning to establish functional connections.
Most embryological malformations occur during this embryonic phase. The remainder of human development, from weeks nine to thirty-eight, is called the fetal period, the time during which the embryo first acquires human appearance.
The medical definition of the fetal period extends to forty weeks because it is measured not from conception but from the onset of the woman's last menstrual period, usually two weeks earlier. Fertilization takes place around the middle of the average four-week cycle, hence the two week discrepancy between the biological and medical gestation period.
A human fetus at ten weeks. The Fetal Period The fetal period is characterized by two processes. The first is rapid growth increase in size and cell number and the second is continued tissue and organ differentiation specialization of cells to perform distinct functions.
Fetal growth rate is greatest at the beginning of the fetal period through week sixteenduring which time the fetus increases twenty-five-fold in weight. The largest increase in absolute weight gain, however, takes place during the final month of gestation.
In these four weeks the fetus gains as much weight grams [a little over 1 pound] as it does during its first twenty weeks of development. Normal full-term babies weigh about 3, grams 7.
Newborns weighing grams or less rarely survive, although medical advances are improving their chances. Infants weighing between and 1, grams 2. Besides weight, the length of the fetus is also used to estimate age and survivability.
Specifically, the obstetrician can use ultrasonic probes to determine fetal crown-rump length, the distance from the top of the head to the bottom of the rump. Many factors affect fetal growth, including the nutritional state and social habits e.
Perinatology is the medical subspecialty concerned with the mother and fetus from the time of viability outside the uterus, generally the last three months of normal gestation the third trimester to about one month after birth.
A perinatologist uses a variety of methods to assess the fetus. Amniocentesis is the technique by which amniotic fluid is withdrawn by a needle inserted through the mother's abdomen into the amniotic sac.
The Fetal Period The fetal period is characterized by two processes. The first is rapid growth (increase in size and cell number) and the second is continued tissue and organ differentiation (specialization of cells to perform distinct functions). Chapter 33 - Toxicology INTRODUCTION. Ellen K. Silbergeld. Toxicology is the study of poisons, or, more comprehensively, the identification and quantification of adverse outcomes associated with exposures to physical agents, chemical substances and other conditions. blood pressure is the force of blood against the walls of arteries. Blood pressure is noted as two numbers—the systolic pressure (as the heart beats) over the .
The fluid itself can be analyzed for various metabolitesand fetal cells that have been shed into the fluid can be isolated and grown in culture for chromosomal analysis.
The fetus itself can be visualized by two general methods. An instrument emitting ultrasonic waves is placed on the mother's abdomen and the reflected signals are rendered into images of the fetus by computer enhancement.These courses are based on a per credit cost and are immediately downloadable.
Note: There are a few courses listed that are not available yet for purchase with the download but you can use the per credit option at the payment button below the Apply Now page and we will then email you the course.
Body The Human Body is the entire structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems.
They ensure homeostasis and viability of human body. Chapter 33 - Toxicology INTRODUCTION. Ellen K. Silbergeld. Toxicology is the study of poisons, or, more comprehensively, the identification and quantification of adverse outcomes associated with exposures to physical agents, chemical substances and other conditions.
The Space Marines or Adeptus Astartes are foremost amongst the defenders of humanity, the greatest of the Emperor of Mankind's warriors.
They are barely human at all, but superhuman; having been made superior in all respects to a normal man by a harsh regime of genetic modification, psycho-conditioning and rigorous training. These courses are based on a per credit cost and are immediately downloadable.
Note: There are a few courses listed that are not available yet for purchase with the download but you can use the per credit option at the payment button below the Apply Now page and we will then email you the course. Vertebrate models (e.g., rodents, swine, nonhuman primates) have long played a central role in biomedical research because they share much in common with humans with respect to genetics, development, physiology, behavior and disease.