Mammals and internal fertilization
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
When you understand gamete structure and fertilization in mammals, view this image and answer question 6.
ACTIVITY 3. DEVELOPMENT OF ANIMAL EMBRYOS
Cleavage to larva in aquatic species
Sea urchin embryos
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
Now answer questions 7 and 8.
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
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.
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.
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.