How to use Blastula in a sentence
Then follows the phenomenon of gastrulation, by which onehalf of the blastula is invaginated into the other, so as to obliterate the segmentation cavity.
The result of cleavage in all cases is a typical blastula, which when set free becomes oval and develops a flagellum to each cell, but when not set free, it remains spherical in form and has no flagella.
When the blastula is oval and freeswimming the inner mass is formed by unipolar immigration from the hinder pole.
Thus a planula larva may be a blastula, or but slightly advanced beyond this stage, or it may be (and most usually is) a parenchymula; or in some cases (Scyphomedusae) it may be a gastrula.
When the blastula is spherical and not set free, the germ-layer formation is always multipolar, either by immigration or by delamination, i.e.Advertisement
If the embryo is set free as a free-swimming, so-called planula-larva, in the blastula, parenchymula, or gastrula stage, then a free actinula stage is not found; if, on the other hand, a free actinula occurs, then there is no free planula stage.
This is the blastula stage occurring universally in all Metazoa, probably representing an ancestral Protozoan colony in phylogeny.