Answer to Question #126835 in Biology for Larry Charleston

Question

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?

Answer 1

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.