Answer to Question #126835 in Biology for Larry Charleston

Question #126835
When observing an embryo as it develops from a blastula to a gastrula. What features would I look for to determine whether the embryo comes from a microlecithal, mesolecithal, or macrolecithal organism. Then, how would I describe how those features would look in a frog, a chicken, and a sea urchin?
1
Expert's answer
2020-08-06T05:27:09-0400

Several features can be used to distinguish blastula and gastrula a microlecithal, mesolecithal, or macrolecithal organisms:

  • microlecithal (sea urchin) - holoblastic cleavage that results in the formation of a pear-shaped blastula with a large spacious blastocelic cavity and a wall consisting of one layer of cells. Blastomers are equal in size and yolk is almost absent.
  • mesolecithal (frog) - holoblastic cleavage that results in the formation of a blastula with a small cavity located towards the animal pole and two-layered wall (epiblast and hypoblast). Blastomers are unequal in size and yolk is moderate.
  • macrolecithal (chicken) - meroblastic cleavage that is restricted only to around blastodisc. As a result, the blastula is represented by a flat disc located on the yolk. The disc consists of two zones - area pellucida and area opaca.

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