Answer:
Sperm-egg interaction
Explanation:
The formation of fertilization envelope is well-studied on the model system- sea urchin. It has been shown that, the fertilization envelope is formed after the initial sperm-egg interaction from the egg surface vitelline envelope and the paracrystalline protein fraction which is derived from cortical granules. Secretion from cortical granules, prevents polyspermy.
Answer:
a. heart development.
Explanation:
As the third week of fetal development starts, the process of formation of blood vessels begins. Also, the splanchnic mesoderm in the head end of the embryo forms heart on days 18 and 19. Since it develops heart, this part of the mesodermal cells is called the cardiogenic area. The underlying endoderm sends the induction signals to these mesodermal cells to form a pair of endocardial tubes that gradually develop into a single primitive heart tube.
This primitive heart begins to beat by the end of the third week of development. Therefore, any drug that affects fetal development during the first 20 days of the pregnancy might affect the development of the heart.
Answer:
Star Creek is flowing from East to West, that is because the creek is small and has just begun to form as an extension of the Cayuta Creek.
Explanation:
<span>An important prelude to a greeting or opening line is often eye contact.</span>
