When the blastula is oval and freeswimming the inner mass is formed by unipolar immigration from the hinder pole.
Next the blastula gives rise to an internal mass of cells (fig.
This is the blastula stage occurring universally in all Metazoa, probably representing an ancestral Protozoan colony in phylogeny.
At one pole only, and in a connected layer with orderly arrangement, so that the gastrula stage is reached at once from the blastula without any intervening parenchymula stage.
When the blastula is spherical and not set free, the germ-layer formation is always multipolar, either by immigration or by delamination, i.e.
The formation of an inner cell-mass converts the singlelayered blastula (monoblastula) into a double-layered embryo (diblastula) which may be termed a parenchymula, since at first the inner cell-mass forms an irregular parenchyma which may entirely fill up and obliterate the segmentation cavity (fig.
Then follows the phenomenon of gastrulation, by which onehalf of the blastula is invaginated into the other, so as to obliterate the segmentation cavity.