The result of this is the production of four nuclei, which eventually take up a position at the bottom of the ovum and become separated from one another by vertical cellwalls; these nuclei divide again, and finally three tiers of cells are produced, four in each tier.
On this view the origin of the sporophyte is looked for in the gradual development of sterile tissue in the generation arising from the fertilized ovum, and a consequent postponement of spore-formation.
They, however, differ in the most striking manner in the structure of the ovum and the early development.
In correspondence with these differences in the ovum there are differences in the early development, though the later stages are closely similar.
The ovum of Hydromedusae is usually one of a large number of odgonia, and grows at the expense of its sister-cells.