Define the blastula and describe the differences between the blastula in different organisms:
a) Blastula is an early stage of embryonic development
b) Microlecithal mammals have a blastula with minimal yolk
c) Mesolecithal amphibians have a moderately yolked blastula
d) Macrolecithal reptiles/birds/monotremes have a heavily yolked blastula
