I'm trying to teach myself the rest of my AP bio book before the exam, and i don't get this stuff on development and reproduction. my teacher said she didn't know because she hasn't had to teach this stuff before, so any help would be great!

1. hows the dorsal lip of the blastopore formed? i don't get it...
2. do only reptiles and amphibians have a gray crescent?
3. why does the second oocyte stay in metaphase 2 until its fertilized?
4. when and where does gastrulation usually take place?
5. whats the main function of the corpus luteum?
6. Is there a difference between an embryo and a proper embryo?
7. If humans have an embryonic disk(like the blastodisc of birds, right?) that cleaves, why are we holoblastic and not meroblastic?

I'm here to help! Let's go through each of your questions:

1. How is the dorsal lip of the blastopore formed?
To understand this, it's important to know that during gastrulation, the blastopore is a depression formed on the surface of the embryo. The dorsal lip of the blastopore is a specific region of the blastopore. It is formed through a process called involution, where cells from the dorsal side of the embryo move inward and spread laterally, eventually forming the dorsal lip. This process is essential for the formation of the three germ layers in the embryo.

2. Do only reptiles and amphibians have a gray crescent?
No, the gray crescent is not exclusive to reptiles and amphibians. It is actually a term commonly used in amphibian embryology to describe a pigmented region that appears after fertilization. However, it is important to note that other organisms, such as some fish and birds, can also have similar structures or patterns during their early embryonic development.

3. Why does the second oocyte stay in metaphase 2 until it's fertilized?
Metaphase 2 is the stage of meiosis II, where the chromosomes line up at the equator of the cell. The second oocyte (secondary oocyte) arrests at this stage until it is fertilized. This is because until fertilization occurs, it ensures that the completion of meiosis (formation of a mature egg) is only triggered upon successful fertilization by a sperm. If fertilization does not occur, the oocyte will eventually degenerate and not progress further.

4. When and where does gastrulation usually take place?
Gastrulation typically occurs during the early stages of embryonic development. The specific timing may vary among different organisms, but it usually takes place after the process of cleavage and the formation of the blastula. Gastrulation involves the migration and rearrangement of cells within the embryo, leading to the formation of the three germ layers: ectoderm, mesoderm, and endoderm. The location of gastrulation varies depending on the organism, but it often occurs on the surface of the embryo or within a specific region, such as the blastopore.

5. What's the main function of the corpus luteum?
The corpus luteum is a temporary structure that forms in the ovary after ovulation. Its main function is to produce and secrete hormones, primarily progesterone. Progesterone plays a crucial role in preparing and maintaining the uterus for pregnancy. If fertilization occurs, the corpus luteum continues to produce progesterone, supporting the early stages of pregnancy. However, if fertilization does not occur, the corpus luteum regresses, leading to a decrease in progesterone levels and the shedding of the uterine lining during menstruation.

6. Is there a difference between an embryo and a proper embryo?
Based on the terminology commonly used in biology, there is no distinction between an embryo and a "proper" embryo. Both terms refer to the early developmental stage of an organism following fertilization. An embryo is an organism in its earliest stages of development, typically characterized by the presence of multiple cells and the initiation of organ formation. The term "proper embryo" may be used to emphasize the specific stage of embryonic development being discussed, but it does not have a different definition.

7. If humans have an embryonic disk (like the blastodisc of birds, right?), why are we holoblastic and not meroblastic?
You're correct that humans, like birds, have an embryonic disk. However, there is a key difference in the two types of cleavage found in these organisms. Birds undergo meroblastic cleavage, which means that the cleavage divisions of the fertilized egg do not occur throughout the entire embryo. Instead, the cleavage is restricted to a specific region, resulting in the formation of a blastodisc on top of the yolk. In contrast, humans and other mammals exhibit holoblastic cleavage, where the entire fertilized egg undergoes divisions, resulting in the formation of a solid mass of cells (embryo) without a distinct yolk region.

I hope these explanations help you understand the concepts better! If you have any further questions, feel free to ask.