(Modified from Horst.) A, Blastula stage (one-cell-layered eaten its way into the in sac), with commencing invaginated endodermal sac, vagination of the wall of the and the cells pushed in with sac at bl, the blastopore.
The four principal phases in the development are: (I) Blastula, (2) Gastrula, (3) Flagellate Embryo, (4) Larva.
Then follows the phenomenon of gastrulation, by which onehalf of the blastula is invaginated into the other, so as to obliterate the segmentation cavity.
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.
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.
This is the blastula stage occurring universally in all Metazoa, probably representing an ancestral Protozoan colony in phylogeny.
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.
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.
The segmentation or cleavage of the ovum which follows ï¿½ upon fertilization terminates in the achievement of the blastula form, a minute sphere of cells surrounding a central cavity.
Thus the development of the two types of individual seen in the Hydrozoa may be summarized as follows: - Egg Free Blastula "Planula" Parenchymula Stage I Gastrula Actinula 1 Polyp Medusa This development, though probably representing the primitive sequence of events, is never actually found in its full extent, but is always abbreviated by omission or elimination of one or more of the stages.