(B) This slit, the dorsal blastopore lip, as seen from the ventral surface (bottom) of the embryo. (A) Gastrulation begins with an invagination, or slit, in the future dorsal side of the embryo. 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.Ĭontinued development of Xenopus laevis. The large yolky cells that remain at the vegetal hemisphere (until they are encircled by the ectoderm) become the endoderm. The cells remaining on the outside become the ectoderm, and this outer layer expands vegetally to enclose the entire embryo. The blastopore expands into a circle ( Figure 2.3C), and cells migrating through this circle become the lateral and ventral mesoderm. Cells migrate through the blastopore and toward the animal pole ( Figure 2.3A, B). 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. This cavity will be important for allowing cell movements to occur during gastrulation. A fluid-filled cavity, the blastocoel, forms in the animal hemisphere ( Figure 2.2H). 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. The sperm and egg die quickly unless fertilization occurs.ĭuring cleavage, the volume of the frog egg stays the same, but it is divided into tens of thousands of cells ( Figure 2.2E- H). Third, fertilization activates those molecules necessary to begin cell cleavage and development ( Rugh 1950). Second, fertilization causes the cytoplasm of the egg to move such that different parts of the cytoplasm find themselves in new locations ( Figure 2.2D). The egg pronucleus and the sperm pronucleus will meet in the egg cytoplasm to form the diploid zygotic nucleus. First, it allows the egg to complete its second meiotic division, which provides the egg with a haploid pronucleus. Other species float their eggs into the center of the pond without any support.įertilization accomplishes several things. Some species lay their eggs in pond vegetation, and the jelly adheres to the plants and anchors the eggs ( Figure 2.2C). Rana pipiens usually lays around 2500 eggs, while the bullfrog, Rana catesbiana, can lay as many as 20,000. The male frog grabs the female's back and fertilizes the eggs as the female frog releases them ( Figure 2.2B). In most species of frogs, fertilization is external. The male leopard frogs make their sperm in the summer, and by the time they begin hibernation in autumn, they have all the sperm that are to be available for the following spring's breeding season. In many species, the eggs are enclosed in a jelly coat that acts to enhance their size (so they won't be as easily eaten), to protect them against bacteria, and to attract and activate sperm. When it has completed this first meiotic division, the egg is released from the ovary and can be fertilized. This is necessary because the egg had been “frozen” in the metaphase of its first meiosis. (B) Frogs mate by amplexus, the male grasping the female around the belly and fertilizing the eggs as they (more.)Īnother ovarian hormone, progesterone, signals the egg to resume its meiotic division. (A) As the egg matures, it accumulates yolk (here stained yellow and green) in the vegetal cytoplasm. Early development of the frog Xenopus laevis.
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