Progeny of 2a-2c and 3a-3d make the mouth and foregut, and the blastopore becomes the opening to the mouth. During this process, several of these cells, as well as the 2d clone, become displaced posteriorly, away from the blastopore. Posteriorly, cells derived from 3c(2) and 3d(2) undergo a form of convergence and extension that involves zippering of cells and their intercalation across the ventral midline. These cells make a novel spiralian germ layer, the ectomesoderm. Anteriorly, cells derived from 3a(2) and 3b(2) undergo a unique epithelial-to-mesenchymal transition involving proliferation and a collective movement of cells into the archenteron. As the blastopore narrows, the micromeres' progeny exhibit lineage-specific behaviors that result in certain sublineages leaving the lip's edge. Initially, descendants of the second and third quartet micromeres (2a-2d, 3a-3d) occupy a portion of the blastopore lip. This is the first study to do so, using the gastropod Crepidula fornicata.Ĭrepidula gastrulation occurs by epiboly: the first through third quartet micromeres form an epithelial animal cap that expands to cover vegetal endomesodermal precursors. Different developmental explanations for this variation exist, yet no modern lineage tracing study has ever correlated the position of cells surrounding the blastopore with their contribution to tissues of the mouth, foregut, and anus in a spiralian.
In this group, the blastopore has been said to become the mouth, the anus, both, or neither. Theories about the evolution of gastrulation often concern the fate of the blastopore (site of endomesoderm internalization), which varies widely in a major branch of bilaterians, the Spiralia. D) alternate between a polyp and a medusa during their life cycle E) none of the above.Gastrulation is a critical step in bilaterian development, directly linked to the segregation of germ layers, establishment of axes, and emergence of the through-gut. Most Cnidarians A) exist only as medusa in their life cycle 8) exist only as polyps in their life cycle C) encyst when they encounter adverse environmental conditions. A Characteristic of hominids is A) larger brain B) blpedalism C) nighttime activity D) monogamy E) both A and B 36. E) give birth to live young rather than lay eggs 35. The monotremes are classified as mammals because they A) are endoderms B) are carnivores C) are omnivores D) have mammary glands. D) control Its buoyancy in water C) tolerate changes in water pressure 34. A swim bladder allows a fish to A) regulate its water content in seawater B) regulate its salt content in seawater C) regulate its salt content in freshwater. The haploid stage in a plant life cycle is called A) zygote B) embryo C) gametophyte D) sporophyte E) conspicuous stage 33. Water conservation in plants is assisted by A) the cuticle B) transpiration C) stomata D) clia E) both A and C 32. Which of the following is an example of osmosis? A) flow of waier out of a cel B) pumping of water into a cell C) flow of water between cells D) both A and B E) both A and C 31. Transcribed image text: 29 Which of the following is descriptive of protostomes? A) spiral and indeierminate cleavage, blastopore becomes mouth, schizococlous development B) spiral and determinate cleavage, biastopore becomes mouth, schizocoelous development C) spiral and deterninase cleavage, blassopore becomes anus, enteroccelous development D) radial and delerminae cleavage, bisstopore becomes anus,enierocoelous developmert E) radial and determinate cleavage, blastopore becomes mouth, schizocvelous 30.
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