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Teves, Mark Joseph p.Bs in Biology 2-3Developmental BiologyJanuary 03, 2021Question 1Question 2Laboratory Activity 6-EmbryologyAre both the egg and zygote of a sea star the same size?The sea star’s unfertilized egg and zygote are not distantly distinct when talking about theirsizes. This is because the sperm that fertilized the egg to become a zygote does not possess enoughamount to make an alteration in the size of the resulting zygote. This statement was supported by VonBaer, founding father of embryology, in which he said the zygote is not preformed from the egg andthese two do not come from an undifferentiated mass.Figure 1.a. Unfertilized egg of a sea star, b. Zygote of a sea star.Images retrieved from The sea star’s unfertilized egg which is also called the ovum has a large and conspicuousnucleus and nucleolus. The fertilized egg or zygote which is diploid in the number of chromosomeshas a less visible nucleus.a. Are the cells of the two-celled stage of a sea star similar insize?After successive execution of mitotic cell division, the zygote is divided into smaller and morenumerous cells. The two-celled stage of a sea star is the product of mitosis that happened in thefertilized egg. As cleavage is occurring, each cell of the said stage gets smaller because this is a divisionwithout growth. The resulting products are two halved cells that are similar in size.Figure 2.The two-celled stage of a sea star displaying the cleavage furrowImages retrieved from a.b.
b. How does the size of an embryo during early cleavage comparewith that of the zygote? Is the sea star embryo growing duringcleavage?c. How many distinct layers of cells do developing sea stars haveat the blastula stage?
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Figure 2.2.The blastula stage of a sea star and its two distinct layersImager retrieved from wikipedia.org/wiki/BlastulationAfter the blastula stage, like any other animals, sea stars also bear three distinct germ layers ofcells during their gastrula stage. These three germ layers are named as (1) ectoderm, the outermostlayer of cells in which epidermis and nerve tissues are derived; (2) endoderm, the innermost layerwhere certain organs such as stomach and lungs are formed; (3) mesoderm where muscle cells andconnective tissue are originated.Figure 2.3.The gastrula stage of a sea star and its germ layersImage retrieved from