Aquatic animals with external fertilization
  Mammals and internal fertilization
  Cleavage to larva in aquatic species
  Cleavage to fetus in mammals
  Flower structure, microspores and megaspores
  Gametophytes and double fertilization
  Embryos of flowering plants
  Germination and growth of seedlings

Mammals and internal fertilization

To complete our activity on animal gametogenesis and fertilization, we will now examine these processes in mammals. Gametogenesis in mammals is unique in that the resulting eggs do not contain yolk (why?). Thus, they are smaller than the eggs of most other animals. Since mammals live on land, gametes cannot be shed into water, so fertilization is internal. As compared to external fertilization, a relatively small number of sperm arrive at the location of the egg, so the block to polyspermy is less robust and slower to take effect.

While all sperm have the same basic structure, there are often distinguishing features in different species. Note the different shape of the sperm head and/or acrosome in these sperm:

human hamster opossum

Did you notice that sperm of the opossum have two tails? This condition is extremely rare and the reason for it is unknown. One might wonder if these sperm can swim. Watch the following video of opossum sperm and decide for yourself.

video - opossum sperm

human egg This light micrograph shows a human egg. It is typical of mammalian eggs and contains no yolk. The first meiotic division has been completed and the first polar body is visible. As in many mammalian species, the second meiotic division will not occur until the egg is fertilized. The covering that surrounds the egg and polar body is called the zona pellucida. It bears sperm binding sites and is analogous to the vitelline membrane of non-mammalian eggs.

Fertilization in mammals occurs internally, as is typical of terrestrial animals. The sperm utilizes the acrosomal reaction to penetrate through the zona pellucida and is then pulled into the egg cytoplasm. After the second meiotic division is complete, the sperm and egg pronuclei fuse as in all animal species.

sperm in uterus fertilization pronuclei
Sperm within uterus
Sperm will traverse the uterus and enter a fallopian tube
Red arrow: a polar body, white arrow: entry point of the sperm
Pronuclear fusion
Circle surrounds the sperm and egg pronuclei

When you understand gamete structure and fertilization in mammals, view this image and answer question 6.


Cleavage to larva in aquatic species

Sea urchin embryos
In all animal embryos, the initial divisions of the zygote are called cleavages. As cleavage proceeds, the mitotic divisions are rapid with no time for cell growth between divisions. Thus the cells become smaller after each cleavage. As the blastula begins to form, the cleavage rate slows down. Future cell divisions are slower, allowing the cells to grow. The formation of a mature blastula marks the end of the cleavage period. Observe cleavage and blastula formation in the video below.

video - cleavage and blastula formation of a sea urchin embryo

When the blastula is fully formed, gastrulation begins. In almost all animal embryos, gastrulation forms a new internal cavity that becomes the digestive tract, and additional cells move inside the embryo to form mesoderm. The mesoderm eventually gives rise to internal organs such as the heart, kidneys, and reproductive tract. Gastrulation is accomplished in various ways by different animal groups. In echinoderms (such as the sea urchin), it is a 2-step process. Study the following video and micrographs to learn how mesoderm and the digestive tract are formed in the sea urchin.

video - gastrulation of a sea urchin embryo

When gastrulation is complete, a mouth forms at the end of the digestive tract opposite to the anus, and spicules (the larval skeleton) are secreted by mesoderm cells. The embryo then changes into the larval body form which is known as a pluteus larva. The larva can swim and feed. After a few weeks of growth and further morphological changes, it undergoes metamorphosis to the adult sea urchin body form. Observe transformation of the gastrula into a pluteus in the animation and further growth of the larva in the micrographs below.

animation (no audio) - development from gastrula to a pluteus larva

young pluteus older pluteus
Young pluteus larva
2-Week old pluteus larva (1 mm in length)

Now answer questions 7 and 8.

Amphibian embryos
All animal embryos go through cleavage in which the cells are reduced in size. In eggs that have a large amount of unequally distributed yolk, the cleavage pattern is asymmetrical and not all cells are the same size. In amphibian embryos, such as the frog, those cells containing mainly yolk divide more slowly and thus are larger than cells contain mainly cytoplasm during most of the cleavage period.

8-Cell frog embryo: " a" is viewed from the top and "b" from the side. Cells containing yolk are lighter in color.

Study the following video of frog development from the first cleavage division to the gastrula stage. Note the similarities and differences between frog and sea urchin gastrulation. Examine the micrographs of the blastula and gastrula stages to compare an external vs. internal view of the embryo. The blastula stage is difficult to detect unless the blastocoel cavity can be seen within the embryo.

video - development of the frog embryo to the gastrula stage

Blastula (external view)
Cut blastula showing blastocoel cavity
gastrula gastrula cut
Gastrula (external view)
Cut gastrula showing new internal cavity

Now we will examine further development of the frog embryo to the neurula stage and then to a hatched tadpole larva. First view the short black-and-white video that shows gastrulation with the blastopore facing forward, followed by formation of the neural tube. Remember that the neural tube is extremely important in vertebrate animals because it forms the brain and spinal cord. Then study the longer color video that begins with neurulation and ends with a tadpole larva. Note that development of the embryo occurs within the tough vitelline membrane which can be clearly seen at later developmental stages. The emergence of the larva from the vitelline sac constitutes "hatching".

video - gastrulation and neurulation in a frog embryo

video - development of a frog embryo from neurulation to a hatched tadpole larva.

Now view this image and answer question 9. You may use this chart of frog development to review the embryonic stages.

Zebrafish embryos
The embryos of fish develop from eggs with so much yolk that cytoplasm is segregated into a small patch at the egg periphery. This is the only part of the egg that divides during the cleavage period. As a result, the developing embryo lies on top of the yolk mass throughout development.

fish 2-cell stage
Adult zebrafish in an aquarium
2-Cell stage of zebrafish embryo

Now view this time lapse video of zebrafish development. The entire embryonic period from the 2-cell stage to larva takes only 48 hours in this fast-developing species.

video - development of zebrafish embryo from 2-cell stage to a few hours before hatching

When you understand the development of Zebrafish embryos, view this image and answer question 10.

Cleavage to late fetal stage in mammals

The development of mammalian embryos has some unique characteristics. Because there is no yolk in the egg, the entire egg divides and cleavage cells are the same size. The blastula of mammals is called a blastocyst. It is unique in that it contains an inner cell mass from which the embryonic body develops and an outer ring of cells, the trophoblast, which will assist in implantation and form part of the placenta. Remember that implantation of the embryo within the uterine wall occurs at the blastocyst stage.

cleaveage cleavage blastocyst blastocyst
Early cleavage
Late cleavage
Early blastocyst
Late blastocyst

The inner cell mass forms a disk which develops into the embryonic body. Gastrulation is a bit different than in the other embryos we have studied, but neurulation is essentially the same in all vertebrate embryos so is like that seen in the frog. View the following animation. It is a realistic view of the human embryo from fertilization to the late fetal stage.

animation - development of the human embryo

Compare development of mammalian embryos to the aquatic embryos that you have studied, then answer question 11.