Describe the stepwise process of
With the help of an example, illustrate how a pair of chromosomes determine the
in of a labelled diagram, explain the life cycle of a frog.
of a child in human beings.
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Answer:
During cleavage, the volume of the frog egg stays the same, but it is divided into tens of thousands of cells (Figure 2.2E-H). The animal hemisphere of the egg divides faster than the vegetal hemisphere does, and the cells of the vegetal hemisphere become progressively larger the more vegetal the cytoplasm. A fluid-filled cavity, the blastocoel, forms in the animal hemisphere (Figure 2.2H). This cavity will be important for allowing cell movements to occur during gastrulation.
Gastrulation in the frog begins at a point on the embryo surface roughly 180 degrees opposite the point of sperm entry with the formation of a dimple, called the blastopore. Cells migrate through the blastopore and toward the animal pole (Figure 2.3A,B). These cells become the dorsal mesoderm. The blastopore expands into a circle (Figure 2.3C), and cells migrating through this circle become the lateral and ventral mesoderm. The cells remaining on the outside become the ectoderm, and this outer layer expands vegetally to enclose the entire embryo. The large yolky cells that remain at the vegetal hemisphere (until they are encircled by the ectoderm) become the endoderm. Thus, at the end of gastrulation, the ectoderm (the precursor of the epidermis and nerves) is on the outside of the embryo, the endoderm (the precursor of the gut lining) is on the inside of the embryo, and the mesoderm (the precursor of connective tissue, blood, skeleton, gonads, and kidneys) is between them.
Figure 2.3. Continued development of Xenopus laevis.
Figure 2.3
Continued development of Xenopus laevis. (A) Gastrulation begins with an invagination, or slit, in the future dorsal side of the embryo. (B) This slit, the dorsal blastopore lip, as seen from the ventral surface (bottom) of the embryo. (C) The slit becomes (more...)
Organogenesis begins when the notochord—a rod of mesodermal cells in the most dorsal portion of the embryo—tells the ectodermal cells above it that they are not going to become skin. Rather, these dorsal ectoderm cells are to form a tube and become the nervous system. At this stage, the embryo is called a neurula. The neural precursor cells elongate, stretch, and fold into the embryo (Figure 2.3A-D), forming the neural tube. The future back epidermal cells cover them. The cells that had connected the neural tube to the epidermis become the neural crest cells. The neural crest cells are almost like a fourth germ layer. They give rise to the pigment cells of the body (the melanocytes), the peripheral neurons, and the cartilage of the face. Once the neural tube has formed, it induces changes in its neighbors, and organogenesis continues. The mesodermal tissue adjacent to the notochord becomes segmented into somites, the precursors of the frog's back muscles, spinal cord, and dermis (the inner portion of the skin). These somites appear as blocks of mesodermal tissue (Figure 2.3F,G). The embryo develops a mouth and an anus, and it elongates into the typical tadpole structure. The neurons make their connections to the muscles and to other neurons, the gills form, and the larva is ready to hatch from its egg jelly. The hatched tadpole will soon feed for itself once the yolk supply given it by its mother is exhausted (Figure 2.3H