Biology Homework Chapter 27: Embryonic Development
Textbook assignment: Chapter 27: Reproduction and Embryonic Development, sections 9-19.
- 27.9 Fertilization occurs when a haploid sperm penetrates the jelly coat, middle vitelline layer, and the plasma membrane of haploid ovum to form a diploid zygote. The enzymes carried in the sperm's acrosome and other chemical signals released when the sperm binds to the ovum's surface receptors to release its nucleus trigger the the separation of the plasma layer and hardening of the vitilline layer, which together create an impenetrable protective shell around the now-fertilized egg.
- 27.10 Cleavage of the zygote produces a ball of cells, the blastocoel. No new material is absorbed during this period, so the increase in the number of cells increases the surface-to-volume ratio but not the volume itself. Cleavage ends when the blastula, a hollow ball of nearly identical cells, has formed.
- 27.11 During gastrulation, the newly-forming cells take on distinct characteristics: ectoderm cells will become the outer layer of the adult organism and its nervous system; endoderm cells will become the digestive tract, and mesoderm cells will become all tissues and other systems, including muscles and excretory organs.
- 27.12 In vertebrate animals, the notochord begins forming within hours of gastrulation along the mesodermal layer; it will become the backbone. Neural tubes form from the ectoderm; somites (blocks of mesoderm cells) initiate formation of the segmented muscles and ribs around a hollow space (the coelom).
- 27.13 Differentiation in genetic expression follows a programmed "turn some on, turn some off" sequence as cells divide, creating different tissues. Programmed cell death causes some tissues to kill off cells in specific regions (i.e., between fingers and toes).
- 27.14 Cell differentiation can be internal, or it can be induced if one cell influences another through hormone secretion and absorption. The homeotic genes controlling differentiation contain sequences that are common in many species, leading some evolutionists to conclude that these sequences were established very early in the history of life.
- 27.15 The fertilized egg of a mammal (in particular a human) is imbedded in the uterine wall. Membranes form a placenta to support and nourish the developing embryo.
- 27.16 The human gestation period of 38 weeks is usually divided into trimesters. During the first trimester, all organs and appendages form; the heart beats and the embryo can move its arms and legs. During the second trimester, the placenta changes function as the embryo takes on more responsibility through its own developing organs; it grows significantly and develops more "human" appearance and proportions. During the third trimester, the circulatory and respiratory systems develop to allow an independent infant to breath on its own. Bones harden and muscles strengthen.
- 27.17 Childbirth begins when the cervix begins dilating (opening up). Uterine contractions push the baby out, followed by the placenta. Once the placenta has detached, progesterone and estrogen levels decrease, causing the uterus to shrink and the pituitary to produce prolactin to stimulate milk production.
- 27.18 Reproductive technology increases options but also raises serious ethical questions about the nature of human life.
Read the following weblecture before chat: Embryology
Take notes on any questions you have, and be prepared to discuss the lecture in chat.
Perform the study activity below:
Use the Guess the Embryo activity to explore how embryos are similar and how they develop during this early phase of life.
- Click on "Launch Interactive" to Start the activity (require FLASH).
- Identify each embryo (don't worry if you get it wrong; this is not a test!).
- When you have correctly identified an embryo, work the through "Watch this embryo develop" slides.
- What characteristics of an embryo might give you clues to the species?
- What differences in embryonic patterns of development and rates of development did you notice?
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery Exercise with a score of 85% or better.
- Optional: Test yourself with the textbook multiple choice questions and note any that you miss that still don't make sense. Bring questions to chat!
- Go to the Moodle and take the quiz for this chapter.
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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