We may distinguish the following series of stages: (I) ovum; (2) cleavage, leading to formation of a blastula; (3) formation of an inner mass or parenchyma, the future endoderm, by immigration or delamination, leading to the so-called parenchymula-stage; (4) formation of an archenteric cavity, the future coelenteron, by a splitting of the internal parenchyma, and of a blastopore, the future mouth, by perforation at one pole, leading to the gastrula-stage; (5) the outgrowth of tentacles round the mouth (blastopore), leading to the actinula-stage; and (6) the actinula becomes the polyp or medusa in the manner described elsewhere (see articles Hydrozoa, POLYP and Medusa).
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.
When the blastula is spherical and not set free, the germ-layer formation is always multipolar, either by immigration or by delamination, i.e.
This is the blastula stage occurring universally in all Metazoa, probably representing an ancestral Protozoan colony in phylogeny.
Next the blastula gives rise to an internal mass of cells (fig.
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.
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.
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.
(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.
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.
Then follows the phenomenon of gastrulation, by which onehalf of the blastula is invaginated into the other, so as to obliterate the segmentation cavity.