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The space occupied by the yolk corresponds to the archenteron or primitive digestive cavity; and the opening at the end to the primitive mouth or blastopore. Ectoderm and entoderm unite around the mouth. Both the blastosphere and gastrula often swim freely by flagella.
This embryo, resembling a hollow rubber ball filled with fluid, is called a blastosphere. It corresponds in structure with the fully developed volvox, except, of course, in lacking reproductive cells. If the rubber ball has a hole in it so that I can squeeze out the water, I can thrust the one-half into the other, and change the ball into a double-walled cup.
A similar change takes place in the embryo. The cells of the lower half of the blastosphere are slightly larger than those of the upper half. This lower hemisphere flattens and then thrusts itself, or is invaginated, into the upper hemisphere of smaller cells and forms its lining. This cup-shaped embryo is called the gastrula. The cup deepens somewhat and becomes ovoid.
The earliest one-celled protozoa were probably succeeded by many- celled animals of the type of the blastosphere, and these by gastrula-like organisms. From the gastrula type the higher sub- divisions of animal life probably diverged, as separate branches from a common trunk.
The fertilized ovum, the cell with which each animal begins its life, grows and multiplies by cell division, and develops into a hollow globe of cells called the BLASTOSPHERE. This stage is succeeded by the stage of the GASTRULA, an ovoid or cup-shaped body with a double wall of cells inclosing a body cavity, and with an opening, the primitive mouth.
Each of these early embryological stages is represented by living animals, the undivided cell by the PROTOZOA, the blastosphere by some rare forms, and the gastrula in the essential structure of the COELENTERATES, the subkingdom to which the fresh-water hydra and the corals belong.
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