The cells themselves, initially in a monolayer, are referred to as the blastoderm . The cavity is the blastocoel . The opposite pole, the vegetal pole , is composed of a smaller number of large, yolk-filled cells. The blastula stage is followed by gastrulation . See Introduction to the skeletal system for discussion of some of the issues relating to formation, growth and morphology of bone.
Vertebrates are unique in having the gills develop internally, as evaginations of the anterior gut.
It is supposed that these internal gills were, primitively, filter-feeding structures which secondarily developed as respiratory organs. The branchial arches support the gills and have also been exapted for various other functions. Plato believed that swallowed liquids went down the trachea into the bronchi. Remember that the next time someone mentions the old fascist. Same as labial and opposite of lingual .