BIO1022 Lecture 1: 999
13 Oct 2018
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Document Summary
The polarity of the egg determines the anterior-posterior axis before fertilization, at fertilization, the pigmented cortex slides over the underlying cytoplasm toward the point of sperm entry. This rotation (red arrow) exposes a region of lighter-colored cytoplasm, the gray crescent, which is a marker of the dorsal side: the first cleavage division bisects the gray crescent. Once the anterior- posterior and dorsal-ventral axes are defined, so is the left-right axis: the blastula consists of a single layer of ciliated cells surrounding the blastocoel. Gastrulation begins with the migration of mesenchyme cells from the vegetal pole into the blastocoel: the vegetal plate invaginates (buckles inward). Mesenchyme cells migrate throughout the blastocoel: endoderm cells form the archenteron (future digestive tube). The gastrula has three germ layers and is covered with cilia, which function in swimming and feeding. 1. gastrulation begins as cells move inwards to form the dorsal lip of the blastopore in a process called invagination.
